Because of AOL's funky proxy servers, it's pretty tough to get a figure for "unique" visitors on anything but perhaps for a rather low traffic website.
But looking at a medium traffic website (about 10mb of Apache logs per day) I see everything from AOL 4.0 on up almost every time I check for AOL. So there's a lot to take into account, if you care to.
I must admit to being lazy on this issue. I figure if I write basic and valid code, then it's AOL's business to give their customers a decent browser. But I also don't get into very elaborate DHTML or anything like that, and I've yet to have any horrible surprises when I do get around to checking AOL.
I gave up on their image compression a long while ago.
Because the AOL browser was a modified Internet Explorer through version 7.0, I'm guessing that multiple versions on one machine would at least require a hard drive partition, if it's indeed possible at all.
But, hey, I'd love to be proved wrong on that one.