I have always bathed Punkin in the tub. I use a sprayer though. She likes the water spraying on her, and tolerates it really well. She gets a treat when she gets out and a play with her fav. toy. I did put water in the tub once when it was really cold out, I thought the warm water would help keep her warm. She promptly peed it it!
That was the last time we did that. It can be hard on the back, but I find it doesn't take too long. I usually kneel on a rolled up towel to help with the kneeling.