Actually, the tip can be quite different paying OOP vs. DDP. If you are going to order all the parts of the DDP anyway, then it is the same, but if you don't want to eat the way the DDP dictates, it does make a difference. For our family, we would not order four desserts; we would order one. Giving the desserts a $7 value, that's almost $4 in tips for every meal. That's an extra snack for someone.
One more. For our upcoming trip, DH and I are doing a date dinner at Skipper's Canteen. If on the DDP, we'd have to order two entrees, drinks, and desserts to make the plan valuable. What we really want are several of the apps and maybe one dessert. Total for what we WANT is $46, so about $8.30 in tips. If we were using the DDP, the very least amount would be $56, so a tip of $10. And I guarantee you, we wouldn't be ordering the least expensive entrée if on the DDP, so our total would be closer to $80 and a tip of $$14.
Just wanted to show that paying using any dining plan will probably increase your total tip amount.