I never have, for anyone. The way I see it, that's my private time. He would like me to, and maybe someday I will... but I have terrible performance anxiety, and just can't get comfortable with the thought of putting on a show.
It started years ago when I had to show him how I like to be touched (I have very particular wants and needs!), next thing I knew I was in the throes of a climax. It has been a part of our repertoire ever since, usually ending with him ejaculating over my fingers while I play with myself.
When our relationship was long distance, we would do it on webcam. That was VERY sexy back then! Now, in front of him... well, it's awkward. We haven't done it too much, but I think maybe it'll pass if I do it enough to allow myself to get used to it.
If it makes you too uncomfortable, don't do it! But you may find it worth exploring at some point.
Don't be shy! Your partner thinks you are sexy and will definitely be really turned on to see you do this. I have done this with my past partners, and it actually INCREASED my confidence, seeing how much they enjoyed it.
I like to if the guy doesn't get too pushy telling me what to do or how to do it. I've had that happen and then it just feels like I'm doing a performance for his benefit, and I can't really come then.
On the flip side, I've really loved watching boyfriends masturbate. Although I had one who seemed embarrassed to do it, so I didn't ask him again.
I've done it and never had a problem. He's not really into it. I think he likes the idea of me masturbating, but he never really asks me to, he wants to do it for me. I have no problem with that either. lol