I don't have an agent yet, so feel free to disregard my advice. Maybe it's just me, but I couldn't think of sending anything out, even to an agent I've known for years, without running it past my wife. She's read just about every book in the English language, and many in lots of other languages, too, and I respect her opinion on literature more than anyone. She has a great feel for my voice and style, and probably knows my work better than any agent or editor could. To me, she's not just some beta reader, she's an extension of my internal editor, and sees things in my writing that I never could because I'm too close to it. But the thing is, I think I've gotten to the point where my writing is polished enough on my own that her comments are probably a lot less intensive than most betas around here. My last story, her only comment was "it's fine," and before that was "i didn't sympathize Kate; make her more likeable," and before that was "it's fine," etc.
So it depends on you and your betas. If you have a very close beta whose opinion you respect more than life itself, definitely show it to him or her first. If it's just a friend who read your book and told you what he or she thought, then you should probably just go ahead and send it to your agent.