Just my $0.02, but I think you need to tip buffet waitresses just as well as other waitresses. If they’ve done their jobs! If they keep your dirty plates cleared, get you new plates, and refill your drinks!
I was a waitress at a chinese buffet place, and I know I only made $2.15/hour on my paychecks. I depended on the tips to supplement my income!!!
Be kind to your waitresses!