I think it depends on the buffet. We just came back and did a few different buffets and one "family meal."
Now, DBF tipped 20% everywhere we went, regardless. He is a server himself (not at a buffet), so that was his mind set.
There was one buffet we went to that the server didn't need to do much-he brought us drinks but we only asked for a refill once. I believe they had separate bussers at this place so his tip was definately shared with them.
We went to the Biergarten and our waitress busted her @$$ I would say way more than a table service server between all the beers (huge & heavy to carry!) and lots more plates to clear from our table of I think 8 people. She was working nonstop.
Then there was the server at our "family style" meal who did the bare minimum and IMHO did not work to the level she should have. We had to flag her down each time to get anything refilled and it did put a damper on the point of "family-style."
So, I think it varies on your experience there and the format of each buffet.
I, personally, didn't mind DBF tipping the full 20% wherever we went because I felt the service was excellent. My only exception would have been the server from our "family style" meal who hardly paid attention to us.

BTW: Is that auto-gratuity in effect for parties of 6+ at buffets too or just full table service?