You can pay for everything with your KTTW card, but a lot of people like to bring some small denomination cash for tipping room service instead of signing a dollar or two here and there. We found that having a small stack of $1's that we put where we all knew where they were was good because then if the kids ordered cookies or Mickey bars from room service, they could give the tip.
But other than that,
DCL is a cashless environment.
I was reading on AAA that they give a 10% discount in the shop on DCL ships (Treasure Ketch particularly). Anyone ever used it?