I admit it -- I LIKE to call 90 days out and set those PS up! There's just something warm and fuzzy about knowing that somewhere, in the annals of Disney's database, my name and party size is registered and they plan to accommodate us on those given dates!
We usually eat one PS meal per day of a given 9-14 day trip. SOmetimes we may even eat 2 (a character b'fast and sit-down dinner). With younger kids, you can't wait until 8 pm to eat. Each person in our fam makes a "request" for a fave or new place to try, and PS are one way I can show that each and every request has been duly honored in the planning!
I would say calling day-by-day works best for those flexible in their meal times, with a smaller party size (like 2 or 4), and outside the "peak" Disney attendance seasons (like xmas,
MNSSHP, spring break, etc). We usually have anywhere from 5 to 14 in our group -- very difficult to be "flexible" !
We did try on our last two trips to change a ressie due to scheduling or change in plans... it didn't work out. Calling a day ahead, couldn't get anything for our party of 8 for most anywhere we tried (Cape May, teppanyaki, LeCellier, WCC, to name a few of the ones we tried). Ended up at GG in the land on one trip, which was ok, but some folks in the party really had wanted to eat elsewhere. On the other trip, called, nothing available (or at least, not before 8:30 pm) so we ended up waiting in line at B&C at 5 pm that day. Granted, it was quite good, one of the faves of the trip, and it doesn't take PS anyway, but the kids would have been happy just to visit there at a less crowded time for ice cream and have had their dinner more "planned and punctual". My kids ask me well in advance of our trip, "Where will we eat? When will we eat at Chef Mickey's?" Even DH asks. Maybe we will plan less as the kids get older and can flex their schedules a bit more... but I doubt it. We're just plan-happy diners!