Ditto. The Keisei-script is super helpful for better understanding and predicting on’yomi. It made a huge difference for me.
On a similar note, @avotnam666 you might want to install
It’s also only used during lessons, but it gives you a bit more explanation for when rendaku happens or not.
Good luck with your studies! 
Edit:
Regarding scripts in general: install when you start to need something in particular from WK. Chances are, there is a script for it.
But, before that, there’s not much point to it. I did vanilla WK for at least 10 levels in the beginning and that was also before they got the review forecast up on the dashboard. Then, bit by bit, I installed stuff to help me attack leeches, or got myself the Heatmap and Ultimate Timeline scripts for additional overview of my review flow. It just made sense as I got further up in levels. It’s fine to leave that for later as you’re still at the beginning of your WK journey. 