I see it this way; it is not really a question of asking permission, it is more of how people view nude modelling in general.

Of course I would ask permission and discuss it with my partner first, whether he be Dominant or not. With a D/s relationship, it would depend on the type of relationship it is. If a Dominant completely owns his slave, then it is only by protocol that the slave ask permission first. Then again it is really common courtesy, just to do so.

This is more of a question in my mind of the misconceptions that people have about nude modelling.
The naked human body, is in fact a beautiful piece of artwork in itself, a raw form of expression. Artists who are serious about their work, will view the human body as such.

People tend to overlook the artistic expression and lean more towards the offensiveness of posing naked in front of a group of strangers.

I am not into art all that much, but from what I have heard, artists derive artistic pleasure from thier work, they are not there to fulfill a deviant fantasy, or to go home and masturbate over thier work later on.

This isn't really the same thing as a man asking a female to pose for pictures, that he will post on his personal website or for fapping material for him and his friends.

I would be very surprised to hear that a model or any artist would become sexually aroused during a session.

For me I would be honoured to be able to use my body in such a way for the artistic pleasure of another.