Having been in the restaurant industry for ten years, I can honestly say this. Food servers (whether they're bad or good) do not have to be tipped. This is purely optional on the customer, but for the server to just make minimum wage is not enough to live on, so if they can score a couple of hundred a night in tips, then that helps them greatly!
I had friends who worked in the various Disney restaurants back in the late 90s/early 2000s who were making some pretty good money! One of them convinced me to jump back into the industry (after already being out of the restaurant industry for 5 years at that point), but since then they all moved on to other careers (outside of Disney) and when they come back to vacation with their families, they tip rather substantially. In my case, I tip rather substantially on the
TOTAL! If you are using a discount card (DVC Membership Card, Annual Passholder, etc), then you are to tip on the total
BEFORE the discount is applied.
In the states, some restaurants may apply a gratuity to a bill and that has to be told to the customer up front either by the server, manager, or even in the menu somewhere. For example, my brother and I went to Bongo's (Disney Springs/Downtown Disney) on Sunday night (9/6/15) and ran up a pretty decent bar bill. When the check came out, we noticed a gratuity line item that was 18% of the bill. This is a guaranteed tip for the server. If we wanted to tip more, then that's on us. But, the 18% in that bill was enough for the server and the server was good enough to deserve the 18%.
So, a friendly piece of advise for our European friends who come over and spend their hard earned dime at Disney - just keep in mind that the folks who work in the restaurant industry work as hard as they can to earn that tip! They're working people and they have families to support, roofs to keep over their heads, bills to pay, etc. Even if your server is having a bad day and you're still enjoying your food and ambiance of the restaurant? Throw a little something their way for trying.