Tips are for all meals, whether you eat them in the buffet, sit down, or pay restaurants.
So, even if you were to have dinner in the buffet all 7 nights, someone will be serving you on the buffet line and cleaning up after you from your table for dinner (as well as for lunch and breakfast). So, I'm not sure why you'd worry about reducing tipping. Just consider it a "done" part of cruise fare, and then ask for the service you need on board vs sitting back and then worrying you won't get it.