This kind of depends upon who you ask.
Here.... it would never be an option.
Much the same way I do not work with a client standing behind me "dictating changes". There is no feasible reason why a client needs to be that close. If they wish to micro-manage to that degree, they need to hire an employee. As a freelancer, it is asking far too much in my opinion.
In addition screen sharing, basically gives the client insight into how I work or what tools I use, and if I'm not careful, personal items I do not want on display to clients.
No. I would not permit any screen sharing at all from my system. They may as well ask to sit on my lap while I work. To me, it is just as ridiculous of a request.
If more immediate changes are necessary due to a looming deadline or something, then a phone call and revisions while on (speaker) phone may be permitted on very rare occasions. With email used to send revisions. This provides a "wall" between what I do and what the client sees while still providing instantaneous feedback if warranted.
But, again, that's my opinion.