Well, in general playing a character who's an exception to some of the rules of society, many players fall into the trap of 'He's an exception to ALL THE RULES!,' or make the character quite one-dimensional in only defining that one charactaristic.
Not that that's inherently a bad thing, I just tend to discourage it personally.
Like, when 3e first came out, there were stories of half-orc bards and dwarven wizards and gnomish paladins...things that people did mostly because the rules didn't say you couldn't anymore. While probably fun to play, your character should be deeper than "He Doesn't Fit the Stereotype," IMHO.
It's fine to have a character that doesn't fit the stereotype. But an orc straight outta the Monster Manual can do that. If here's no other reason to make him charismatic than the fact that most aren't, he's not that compelling on anything more than a novelty level.
Of course, there are places for charsimatic orcs...I guess the thing to realize is that just because you're playing a "one in a million" PC, don't expect people to think you're special for doing it. A charismatic orc might get a double-glance from some people, but most of the time it'd be no different than having a charismatic elf who happened to be beefy and dumb.
I'm probably babbling.

Basically, as long as the guy is more than "Orc with charisma!", it'll be fine. The danger is if he's nothing else.
