I have to disagree with that. At times you remember that a word is specifically on’yomi or kun’yomi, but you forgot which one is which. In that case it is good to have your mnemonics tied to if it’s on or kun.
Besides, being able to make an estimated guess on unknown words is very useful. Language is not about being flawless, it is about getting a message across. And especially with typing or looking something up in a dictionary it saves time if you can directly use the right *yomi.
@langbot I tend to look at the list of vocabulary that the kanji is used in. That usually gives you an idea about if you are learning the on or kun readings. Perhaps useful for later levels is also this user-script: [Userscript] Keisei 形声 Semantic-Phonetic Composition
Edit: Another useful userscript: [Userscript] Wanikani Katakana For Onyomi
It is quite common that on’yomi readings are given in katakana for dictionary purposes. This userscript creates that distinction in wanikani (tsurukame also has an option for it).