Well, I've never really had "BAD" service at WDW!
And yes life was easier when the gratuity was included. Which some servers told me they got approx. $5/head . . . so overall that is NOT bad.
They do leave a blank line at the bottom of your receipt, and you can leave what you feel is right. They show options for 15% and 20% so you don't even have to do the math (this is before tax) which makes it easy.
I'm pretty much a 20%+ tipper. Being a server is a tough job, and the general public is grouchy!

You'd have to REALLY do a bad job for me not to tip, or give less.
AND-really with the tip NOT included servers are getting the short end, because you get two tabs at the end of the night, your food and your bar . . . and you really should leave a tip on both. WITH the "new" DDP there is so much paperwork at the end of the meal, it is sooooo easy to get confused and tip too little or TOO MUCH . . . which we did one night! (luckily we had an honest server who showed us our error)
