Compared to WDW there really aren't too many sit down restaurants that even take PS. If you want to dine at Blue Bayou you definitely need a PS, and I would also make one if you want Minnie & Friends breakfast at the Plaza Inn. In DL, for Carnation Cafe and Cafe Orleans you might be able to just wing it being mid-September. But, if you want to be sure, it doesn't hurt to make your PS ahead of time.
DCA is now down to only 2 sit down restaurants. Due to the lack of choices, and the popularity of Ariel's (despite the consistently mixed reviews) I would also plan ahead here.
We are making our first foray into the world of the hotel restaurants during our upcoming trip. I made my PS 60 days out for both Storyteller's dinner and PCH Grill Lilo & Stitch breakfast. However, I needed to change the breakfast time a week or so ago and had no problem.
Personally, I am kind of a control freak and I want to know that I have the PS I want, when I want them. We hit a situation a few trips back where the Carnation Cafe got closed due to weather and thus they cancelled our ADR on a Saturday night during the holiday season. We had a VERY hard time getting something else set up in either park, the hotels or DTD. Luckily the staff of Cafe Orleans took pity on us and found us a table for our small little party of 2. If your group is larger, it may be even more difficult to find something on the fly.
That being said, there is some decent CS--Plaza Inn, Red Rocket's, and Pacific Wharf to name a few that serve much better than the standard CS fare of hot dogs and hamburgers that seemingly have totally overpopulated WDW!