I tip 20% for a good waiter at TS meals. Good waiter does not always translate to good service (though in my mind they can make up for delays and other indicators of bad service) So if the service was lousy, yet I didn't come away thinking it was the wait staff's fault, I wouldn't stiff them.
I've been lucky to have never experienced bad service at Disney though. Closest to average was Ohana's (didn't come back to refill drinks that often - and I'm not asking to have my drink refilled every 10 minutes), but they were still average and I gave them 18%. I had a bad meal at Skipper's Canteen, but the waitress herself gave good service and tried to be engaging, so she got 20%. The fact that I didn't like the food should not affect her tip. I travel with family and my brother-in-law is very into fine dining, so I tend to book 2 credit meals, though I have eaten at a handful of 1 credit places. Wonder if this makes a difference?