Never...unless they roll in something really disgusting. They don't go to the groomers either.
We don't go to the groomers. I took Hudler once as a puppy and it scared him (he hates the hair dryer, and I think they stuck him in a crate with a dryer blowing on him).
I find that as long as the dogs are well socialized in all areas as possible, you don't necessarily have to expose them to every situation known to man. I rarely put my dogs in the bath. But I had to soak feet a few summers ago due to infections. I put the epsom salts in the tub and put the dog in the tub and sat there for awhile. They weren't HAPPY, but it's not like it was hard on them or freaked them out. They trust me pretty much.
Our previous dogs have all been shelter dogs and the only time we've bathed them was within a week or two of adopting them (to get the shelter smell off them) and only when they were exceptionally stinky or dirty. I don't think it's necessary to do it more than that.
Emmett had his first bath at 12:30 last night... because he thought it would fun to walk under big brother while he was peeing... So the pee head puppy needed a bath and all I wanted to do was go to bed. He did real good but was not overally happy about it.
Warning... He was then full of energy for a good hour. He ran the U shape of the bed at least a dozen times.