You are absolutely right that is is riduculous to make PSs at the 90 day mark when the parks don't post hours until the 60 day mark.
I personally didn't wait to make my PSs. The only ones that would be affected were dinner reservations and that would only happen if you wanted reservations later in the evening. You can still make breakfast and lunch arrangements.
I've made all of mine and the only one I have to call back on is my dinner at LTT. I think my reservations is at like 5:45 and I want it for a little later, but that was the latest they could give me until the hours are posted.