The only prepooing I do is when I DC on dry hair before I shampoo it. But that's only because that's what the directions say is the way to use the product for deep conditioning (AO GPB), otherwise if not using AO GPB to DC, I normally don't prepoo. For some people it is an important part of their regimen; for others not so much.
OP try one of the suggested methods and see if it makes your regimen better. That's how to know if you need to be doing it. Some people prepoo overnight using a baggy (plastic cap, plastic bag, Saran wrap) to keep the product from getting on their pillow.