As a party of 5, we didn't "get what we wanted" last minute. I made ADRs early and canceled some the day before because of how our schedule was going. I wasn't successful getting day-of, at all. That was ok, I had little expectation of doing so but I'm glad I didn't try that for every day. CS worked out for us when TS didn't. Places that were hard to change or get last minute were Sci-Fi, 50's, Via Napoli, Boatwrights (very surprising), and Homecoming (never worked out over 3 evenings of trying). One night we ended up at DS without planning to and Raglan Rd was the only one that had an opening during our dinner time frame. Another night I tried to change the time for Trail's End and couldn't. This was in early June and the parks weren't busy, yet.
I'm not planning very many for my upcoming trip in January. We're going to take advantage of lounge apps/drinks more often than full meals. Even though we'll be two and don't anticipate difficulty, I'll make a few at 180 so that I don't have to wonder about availability. Also, I find it easy to work around meal times when touring.