Most restaurants in WDW will show a suggested amount now I believe, but you are not bound to it - tip as you see fit. We have never had service in Florida that I have found to be sub-standard and can honestly say we have never tipped at least 15%.
Please be aware that some restaurants in Orlando will ADD the tip automatically so please check your bill before leaving a tip.
Many years ago while visiting Ming Court on I-Drive, my parents gave the waiter a nice tip along with payment of the bill. He returned as we were finishing our drinks to try to return it and explained that a tip was already included in the check. But my mum explained that she wanted him to have the extra as he had made our time there simply outstanding.
In another Chinese restaurant, my mum paid the bill by card, but left a cash tip on the table when they left. The waitress was not happy that there was no tip added to the credit card slip so she actually followed them outside and asked for one!

My mum was horrified and told her to go and take her tip from the table. My mum also told the waitress that if she'd asked for a tip before they had actually gotten outside, she would not have been getting one!
If you experience really bad service, don't leave nothing at all - leave a cent - that way the server will know you were very unhappy with their "service".
