I don't think it's a guarantee you have the same person all the time. We did in-room dining in a 1bd on the Fantasy last year and while it was the same person most of the time, it wasn't all the time. So if you want to tip someone, I'd do it during that 'occurrence' as opposed to waiting until the end.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!