It's hard to generalize, because it depends on (1) what hours you plan to be in the park and (2) what attractions you want to do (and do you want to do any of them more than once)?
You won't need FP+ the first two hours the park is open. But then, the earlier you get them, the better the chance for a 4th/5th FP+ that will be worth something (Magic Kingdom is really the only park where they are likely to be worth something). On the other hand, if you save the FP+ for the afternoon hours, you will be using them during the busiest time of the day (which is definitely when you don't want to be standing in standby lines, except for the shows/rides which never have much wait).