Not as much as you would in typical restaurants where the wait staff waits on you but buffets still have people watching over the dining room, picking up dirty plates, cleaning tables, refilling drinks, etc... So yes, you still tip.
i worked at a buffet and made better tips than i did at any full service restaurant. if there is 2 of you, and they get your drinks, and clean tables, 3-4$ or a percentage of your bill.
i on avg. made 4$ a table if i recall correctly