Yikes. I almost answered your question wrong because I thought you were asking about tips for the Disney Dining Plan (DDP). Good thing I reread and saw that the question was about the Disney Dining Experience (DDE) card, discussed
here.
WIth the
DDE (rules are different for the DDP), Disney does add a gratuity fee of 18% to all bills.This 18% total is calculated on the total price of the food and beverages. You do not tip on the price of your meal taxes.
18% is a standard tip level for good service. The etiquette books I consulted recommend 15% to 20%, with even more for extraordinary service or an elegant restaurant. I think 18% is a fair gratuity for the service you will normally find at WDW. If for some reason you have terrible service that you didn't feel warranted an 18% tip, it would be perfectly within your rights to speak to the restaurant manager about having the tip removed or reduced, but I very much doubt that this will be the case.
If you were paying cash for your meals without the DDE, you would still need to add a gratuity. Let's say I'm paying cash and my meal costs $100. I will then add a tip and pay the restaurant $118. If I have the DDE, my meal costs $80 ($100 minus a 20% discount). Then, when I add an 18% tip (based on the original meal cost), I end up paying $98. I've saved $20! I would rather pay $98 than $118 dollars for the exact same meal.
While the DDE card does cost $60 for annual pass holders like me (prices sometimes change), you can see that in just the example above, I saved $20. Four more meals like that and I've made back my investment and the real savings begin. During my most recent WDW vacation, we were a party of seven that ate lots of table service meals. We found our DDE savings to be
very substantial.
Hope this helps.