We always tip, no matter where we stay. We leave a tip each morning on a pillow. This has ALWAYS gotten us better service no matter where we have stayed (WDW or otherwise).
Our best story was our first trip to WDW when we stayed at POR. Our youngest DD was in kindergarten, and had brought about 6 or 7 dolls (barbies, magic attic, etc.) with her in a laundry basket. You can only imagine the look on her face when we came back from the park for an after lunch swim and some down time. There, on her bed, were all of her dolls arranged in a circle, with some Disney china - having a tea party!!! Every day, we came back and the dolls were doing something different (one day they were watching TV, another day three of them were getting ready for a bath, etc). It was soooo special. I would like to think that that is the kind of service Disney gives, tip or no tip, and is one reason why so many of us keep coming back!
We have always had clean rooms, plenty of towels, soap, shampoo, etc and have no qualms about tipping - I appreciate the fact that someone is helping me enjoy my vacation by cleaning!!!!