One thing to bear in mind here is that the selection of a given FPP affects the desirability of subsequent FPPs. Say, for example, the optimizer tells you that Splash and Space Mountains would be good FPP selections. If you put in just Splash, you may find that after reoptimizing it has you hitting Space at RD so you don’t need an FPP.
So, to answer your question, you should reoptimize after entering FPP reservations to see what effect they have on your plan before actually booking them in MDE. Working with optimized plans and FPP reservations can be tricky, so check out Making Optimized Personalized Touring Plans with FastPass+ for a strategy and tips.