i wouldn't think it'd be wrong per se. i think i'd just rather that someone who was interested in doing that do as they guy Dave referred to- just hold a sign up letting people know that you're available to answer questions. actually getting in the face of people while they leave church is the same kind of thing that the religious do that so irks people.
besides, you're much more likely to get people to ask you questions (the guy Dave referred to has a phone number and an email address on his sign) when they don't have to travel to some venue to attend a discussion.
finally, in a situation where it's several Christians against you there's the very real probability that things will just be a bit of a free for all. it'd be much better to have more one on one discussions with people i would think. that way they could be more honest about their thoughts rather than having to say what's expected of them by their fellow congregants.
that sounds like that's pretty much what the dude that Dave was referring to does. he has a sign he puts on the roof of his car and recently i saw that he was thinking of putting fliers on churchgoer's cars.
It's not wrong. I just wonder if proselytising might seem annoying. Isn't why people hate jehovah's witnesses and mormons or any of those evangelicals who hang out on street corners like their god's chosen.