We have a lot of guys that shoot as they carry; meaning an IWB, a loose shirt over, maybe a long pull polo shirt untucked. As James said, you won't win any matches shooting like that, but you are practicing with how you normally carry. There are some holster rules for safety: strong side holster behind the center line is the main one. That eliminates cross draw, appendix carry, shoulder holster, etc. Other than appendix carry, those methods will have you sweeping your muzzle across the safety officer and/or the other shooters as you draw. I'm not sure the rationale behind banning an appendix carry and there is consideration to allowing it in the next rule book. One guy that shoots with us has a "weak side" gear set - he's right handed but shoots a match left handed once a month or so. That's a great idea and I will be getting some left handed gear as funds allow.
Another issue that's different between "gamers" and "martial artists" as James called them is in the use of cover. You are allowed per rules to expose quite a bit of yourself - your lower body must be behind cover, and half your upper body. If someone is shooting back at me, I am going to try to get as much behind cover as I can.
Movement is another item - there's the "IDPA Shuffle", where you move very slowly taking tiny shuffling steps. Guys do that because they are more accurate, and after they have fire the number of rounds they want they then run to the next place. Obviously, if you are being shot at you need to move as quickly as you can. That can cause problems for the martial artists as if you move quickly, you often can't get aimed shots completed before you run out of room. (At least I can't, anyway...
) Still, there aren't many places where you can shoot as you move, so any practice you get here is helpful.
While a lot of the gamers will give you tips on how to score better, if you let them know your goal, they are very respectful of that. You can get a lot of tactical tips too, once it is known that is what your looking for.
