For CRT breakfast, yes, you'll need one. And with that restaurant, the PS go in minutes when they are released 90 days out, and some people may never get a hold of one. There is a credit card deposit on it as well, so no shows are very very rare.
Depending upon the Epcot restaurant and time of year, you might be okay. We've walked into Marrakesh without a problem, but others like Le Cellier are more popular and can run the risk of filling.
If you want to do a character meal, I would highly advise a PS. It's fairly easy to figure out which park you'll be at each day based on park hours and what you'd like to see, so you can just plan those specific meals for those days. Places like CA Grill, too, and 'Ohanas can be packed and not allow walkups at all. We've even eaten at PT Cafe and they were telling people no walkups that evening...
And remember that for most meals, you can easily cancel (or just not show up, though it's nice to cancel) if you decide to do something else. We've done this before, had a PS in MGM, but got tied up in Epcot and just stopped by and canceled our PS.
Most PS can be made 90 days out, and the hard to get character meals will be the ones to really try for that far out. Others are pretty easy to handle.