Tipping is discretionary. I get that. A 15-18% tip for decent or better service is nothing to sneeze at. But in the context of talking about an expensive WDW trip, and what I can only imagine is a ‘not cheap’ 10 day Caribbean cruise, the first and only line item that you mention skimping on is the gratuity for your servers?!
If, say, $1,300 is spent on meals; the difference between a 15% tip across the board ($1,495) and a 20% tip across the board ($1,560) is quite negligible in the context of cost of multiple trips, but may well make an impact on the servers.
EDIT: In addition, it does not appear that gratuity is included in DDP:
https://plandisney.disney.go.com/qu...lan-says-gratuities-not-included-mean-555713/