You do not have to tip the butler each time. Also, sometimes the butler doesn't bring room service-depending on the time and what you order ( it could be regular staff)..then I would give that person a tip..
On O the gratuities are automatically added to your ship board account if you want to add or subtract from them you just go to front desk and fill out the form to do that. Nothing is written in stone.
We usually do tip butler extra at the end, because we like having cocktail parties and your party setup in cabin can be extraordinary. While not all off them will go this far out, we had mentioned to ours before going out one morning that we wanted to have a party in the cabin before dinner for friends. When we returned he
was waiting there and went so far as to scattered `rose petals' about and had tables of appetizers ( we hadn't asked for all that but he felt it would be festive and the goodies were perfect)and then asked if there was anything else.."can we please take you home ?" was our answer..so while they all do a great job..some go above and beyond our expectations.