Well, I would just emphasize something I said earlier, which is pretty similar to JimBob's interjection: if you find yourself getting into these arguments frequently in the course of actual play, then there's a good chance you're a jerk.

Another thing to consider is that playing cooperatively when a group really wouldn't mind you playing competitively isn't all that harmful. I think the reverse is. Now, playing cooperatively and then hurting the other folks' fun by being a "bad sport" is harmful. But unless the game starts off with full-on competition, you're not likely to run into that without fair warning.

In other words--and I think this could practically be proven mathematically--since forming a group is a cooperative endeavor to begin with, then absent explicit rules, full-blown competition is unlikely to manifest itself early on. Instead it's likely to follow a pattern of escalation. (Proof: if PC A immediately slits PC B's throat, the rest of the party will see he's a psycho and off him. So PC A can't afford to do anything too extreme early on, and the group will establish norms of competition that nobody will push too far in one step lest they suffer retribution.)

Full-on competition can stay hidden throughout a game, though, leading in the end to the "cold-blooded double-cross". But if you don't see any minor competition early on, yet you seriously see no justification for believing that everyone expects to play cooperatively, then you should probably be pretty suspicious if any player amasses a disproportionate ability to carry out a first strike. If the other players are allowing same without a peep, then it's quite likely that your assumption is incorrect--they should be protecting themselves and seeking a balance of power if they think competition is an acceptable possibility.

Ergo, if the guy who's amassing disproportionate power is you, you can either keep quiet (but you're probably a jerk) or say something. If the guy who's amassing disproportionate power is someone else, you can either keep quiet (but you're disadvantaging yourself), surreptitiously try to gain power for yourself (but you're probably a jerk), or draw attention to the fact (in which case you've signalled to everyone else that you're playing competitively).