When is self-selection a problem? Mincer equation
There is no fundamental difference between the choice of occupation and say region. In fact,  there exists a large literature in migration that builds on Roy's model, starting with Borjas (1987).  I'd argue that ideally you'd like to correct for self- selection/ sorting in all choice dimension. In practice this can be hard/ cumbersome/ impossible to do.

Why do we tend to worry about self- selection more in the context of occupations than regions? I suppose it boils down to the fact that everyone had to actively choose an occupation at some point,  whereas we are all born in some region. But,  again,  there is no fundamental difference.