If it is a holiday weekend or a really busy time, I say yes! Make at least one PS for each day, we generally make 2, like breakfast and dinner or lunch and dinner. First of all, there are not that many places the do PS in DL; just Carnation Cafe, Blue Bayou, Cafe Orleans and the Plaza Inn (for Character Breakfast Only). There are plenty of self serve, walk-up places but the lines are unpredictable and we have waited really long to get food and find a table.
In DCA, there are even less PS places; Wine County Tratoria, The Vinyard Place(Winery/Viney?), Ariels Grotto(Princess Character Dining) and...gosh is that all? I think so.
DTD and the hotels all have dining places but I would not go for dinner on the weekend at say, Rain Forest Cafe without reservations! I have seen the wait over 2 hours!! I have kids 6 & 11 who would never be able to wait that long for dinner. We like the Character meals at the different hotels too and I have seen them turn people away from Goofy's lots of times so it pays to have a reservation.
That said, you can always cancel any PS you make!! There is no charge to cancel so why not have a "back-up" plan just in case. We are going right before Thanksgiving and I know it will be crowded and also it might rain. If it rains, people pour into the covered dining places and you can not find a table!
Also, if it is really packed, we do not want to leave DL for fear of not getting back in so I also take this into account when planning PS. For example, we would not leave DL to go have dinner in DTD on Thanksgiving Day. I also plan it all out so we are not doing 2 character meals back to back.
Here is our tentative plan just to give you an idea. Friday-breakfast(feed kids in the room-quick and easy while I dress), lunch BB, dinner RFC. Sat breakfast at Plaza Inn (early entry day at 7 so late breakfast at 9:30) no lunch PS, Dinner Cafe Orleans, Sunday breakfast at Carnation, Goofys for dinner. Carnation has a different menu for breakfast than lunch/dinner so you can eat there twice.
This is just to give you an idea but we use it as a "back up" system and if we decide on some other big lunch, we just cancel something else. We have even called the day of for PS for example, we were at the pool and the kids wanted Goofys for dinner so I picked up the house phone and we made PS for an hour later, it was great. One morning they wanted to try Granvilles(now 55 something?) and we called from the room rather than wait in line and got right in!!
IF we didn't have kids, we would just make one or two PS at places we really wanted to go like BB. Forget about walking up and getting in there. With kids though, we like to have a sit down and relax meal and don't want to/can't wait 2 hours!