I'll make PSs for difficult character meals like CRT and SPB, and maybe for a particular restaurant *if* I know we'll definitely be somewhere at a certain time. However, for the rest of the meals, we are pretty much flexible. We've done spring break over the past two years, and have done pretty well with this system as well as during the off-season. As our day or week unfolds and we have a better idea of where we will be, we make our PSs the day before or even the same day, if at all. For example, before leaving MK for a monorail resort, we'll contact WDW-DINE or check with guest services about making a PS so we don't waste a trip to the resort.
There have been times, very few, where we have not been able to get into our desired restaurant at our desired time, but in those instances we've always been able to find an alternative. Places like Le Cellier will actually help you locate reservations elsewhere if you don't want to wait as a walk-in.
Eating at off-peak times helps, too. Just this past April, as we were walking around World Showcase, a CM from Le Cellier was standing out on the promenade drumming up business for the restaurant. Must've been around 5 or so, because during the latter half of our meal, the restaurant started filling up.
We've also had some great spontaneous meals by not preplanning everything. One night we were in FW around 5-6pm, just looking to grab a quick bite in The Land food court. That night, FW was closing at 6 so we thought the full service restaurants also closed. We noticed the Garden Grill was still open with people just being seated so we walked over and had ourselves a great character dinner!
HTH!