Yes, I have done this. All but one time, my therapist volunteered that I could email, with a caveat that she might not have reception at times.

I think it depends on your situation and need. If a therapist offers, then I would take that at face value and make contact if it's going to be helpful. For me, it has worked well and I'm glad I did. Don't judge yourself for it based on what other people do or think. This is between you and your therapist.