Mary, I know of a very successful famous artist who has it in his contract that he is the artist and he makes the final decisions on a painting. He allows the client to participate in selecting a photograph, on which they sign off, and they are permitted input at a clearly specified stage in the painting. After that, he decides if the nose looks like the client's nose! :)

Hard to imagine that he can get away with that, but, hey, when you reach a certain level it's a whole different ball game.

I think the key is in choosing which stage the client can give input. One of my problems is that my studio is in a public location so the client or anyone else can pop in anytime during certain hours. What I need to tell them is that they can "look but not comment" until I am ready for their input.
If I don't agree with their input, it would be very difficult for me to continue my way even if I was famous. It would always bother me if I thought that the customer wasn't happy with a certain aspect of a portrait.
Ah yes, the joy of being an artist who does portraits - yet it is soooo addicting that I love the challenge of capturing a person on canvas!
:)
Mary