Well… as I wrote in another thread recently, I’m probably in the minority here but…
EDIT: I actually did start with the grammar (Tae Kim), but soon gave up because I thought all the looking back and forth between the sample sentences and the vocab list was getting really frustrating. When I came back a few months later I didn’t have that problem.
I personally agree that the primary concern should be whether you can craft a study schedule that’s sustainable, rather than look at WK level.
I started and dropped grammar a few times before I found a combination that worked for me in terms of resources and the structuring of my time.
Try starting as soon as you can, but be flexible and forgiving if you can’t handle wrapping your head around everything at the same time. It’s okay to slow down. It’s okay to put things on hold for a bit. Just try again the next month, until you find a way that works for you. 
I would start with grammar immediately as well, especially if you’re already consuming japanese media like anime.
I’ve heard grammar like “… tsumori” (plan to) or “… yori” (more than) so often that it’s already ingrained, and i wouldn’t have known these words beforehand.
As Lee’s motto in Naruto: “kinou no jibun yori tsuyoku” - stronger than yesterday ('s oneself)
By the way, i recommend Bunpro, it’s from the makers of WK, it’s basically WK for grammar with a similar lesson/review structure, but the progress is completely free.
It’s not easy to do your reviews for both WK and grammar every day, but you can pick a slower pace for grammar, and not worry about getting reviews down to 0 every time.
By the way, WK also recommends to start reading between levels 10 and 20, and you won’t be able to do that without grammar.
There was an insightful comment from a lvl 60 WK user who couldn’t understand most of what a video game in japanese said (World of Final Fantasy), so he quit it. He came back after learning grammar, and finished the game soon after.
When I studied Japanese in college, we spent the first 2 weeks learning kana. Then we dove into chapter 1 of the Minna no Nihongo series. And learned Kanji as we went along.
If you start with Genki, then you don’t even need to know kana at first, since it gives you romaji (still learn kana though)
So when to start with grammar? As soon as you have the time for it. Which I hope is now. You don’t need to know anything to start. Of course it helps, but it won’t be detrimental to your progress to not know anything at first.
Just keep a steady balance between grammar, kanji and vocabulary.
I might add that what I did do (for Japanese as well as subsequent languages) was heavily focused on learning vocab through or with example sentences.
That probably went some way towards internalizing the grammar even though I didn’t actively study it as such.
So that’s an alternative I wholeheartedly recommend, if you feel like full-on grammar studies aren’t for you yet.
(My very favorite method of learning nowadays is an Anki deck with a spoken word on the front side with a spoken sample sentence on the back. Most often hearing the sentence is enough for me to know whether I got the word right or not.)
I did 3 lessons of Minna before wanikani, plus Michael Thomas Foundation, and I was in situation, when sometimes I meet sentences where I can understand some kana words and structure because of Minna and MTF, but almost all kanji were closed for me. Which was depressing a bit.
After a while I started with WK with significant speed (15-20 lessons per day), I stopped every other learning, and quickly enough moved to opposite situation, I know a lot of kanji and complex words, but my grammar and audit become weakest points. That was a moment, when I decided to slow down WK and return back to Minna, and other learnbooks.
I have to say, there is no conflict. You learn new words in WK, then you learn how to use them in learnbook. You learn some new word in learnbook - no problem, in WK it will be like very easy word, or 2 free prononciations for 2 kanji. Both ways combines perfectly, and they together makes good foundation of Japanese.
I think there is no right level where to start with grammar. Constantly try to use you Japanese, constantly test yourself on NHK, anime, manga, chats. Constantly measure your weakest points, and then dedicate some time to cover them, slowing down other parts. Kanji+vocab = WK or Anki, grammar/audit = specific learnbooks.
I learned basic grammar and vocab with JFZ before even starting WK.
There’s no reason to postpone it. Here’s a good way to start
I’m using Minna no Nihongo too and this is how I feel. I started the book before WaniKani but the first 2 chapters were pretty much the extent of my Japanese from previous learning attempts so I already knew most of the kanji. Then I got to about chapter 7 or so I decided to just focus on kanji for a while and I think it was really helpful for me. For that time period, my Japanese wasn’t progressing particularly well since I was just learning kanji and random words but now getting through chapters in Minna no Nihongo is way easier. Another person may not have needed to take a break to focus on kanji, though.
I say learn grammar sooner rather than later but when varies by person. The OP seems to want to start grammar now so I think they should do that.
Yess Tae Kim saving lives out here!
I’m doing the same thing. Tae Kim, Genki, WaniKani, and KaniWani.
I do WaniKani every day, Tae Kim 2-3 times a week, and use Genki as reference.
I’m not the only person who thinks the solution to not having made time for the last textbook is to buy another one? Good to know!
+1 for this 
suggestion to start after WK lvl 20 seems to be because grammar terms appear in the second part of lvl 19.
so, basic grammar can be learnt before that, and lvl 20 is needed once grammar learning material starts to operate with all these terms:
歌詞 形容詞 自動詞 助詞 数詞 動詞 品詞 名詞
supposedly, trying to eat them before proper serving is not good thing…
(though, personal opinion is: learn sentence structure, basic particles as soon as possible)
Pretty sure it isn’t, though?
Also I agree with many people here, start grammar sooner, rather than later. Knowing a couple words before you read them in a textbook is fun, but a textbook will use many words that show up late on WK, too. There will be a wordlist to study if you go the route of a traditional textbook, so you can go into a chapter prepared with the words you need there. There is not a lot of sense in knowing a bunch of words if you can’t put them in sentences.
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