It's really important to tell people what's going on with you, and a group email is a great way to do that. I think you should write a big email telling people when you have work in a show, are in an art fair, or are otherwise professionally engaged. People do want to know these things. I don't think you're pestering people.

I think a really good email blast is:

1. really short and has an image
2. contains a joke if you are so disposed
3. does not start out with an apology
4. Doesn't have 400 addresses in the To: field. Sometimes having contact info out there for all to see is upsetting to people. Use the Bcc: field.

But that's just what I think. I could be wrong. What do you think?