When is appropriate for the boyfriend's parents to meet my parents?

I've been dating a wonderful man for almost 6 months now. He's 25, I'm 23. He's the second serious relationship I've had. At this point, I would consider it in the "serious" category; in other words, we aren't "dating around," we are monogamous, we've discussed a possible future together, what we expect out of marriage, kids etc. I also know he probably means business because most guys his age bail after a month or two if they aren't serious about the relationship/or want casual dating etc. I've met his parents & he's met mine. We've both spent a fairly significant time eating dinners at each others' house, getting to know family members etc. At what point should the parents meet though? We haven't discussed it & I'm not worried--I'm just starting to wonder a bit about it. He graduates from medical school in May. uming we're still together at that point, would it be a good time for the parents to meet (as I'm sure my parents would want to go to his graduation)? I know that graduation can be a hectic/stressful time for parents though as they prepare meals, get a party ready etc. So I'm not so sure if that would be ideal. Is it a good/bad idea to wait for them to meet til you're getting engaged? I know some people who have done that. Any input on this appreciated! Just don't know the etiquette on this & how to go about it. I think they'll all get along well & can't wait for them to meet. Looking for mature/experienced help..hence the reason I put this here. :o)