It falls more on the common sense rules of life. You are not trying to hide or disguise the weapon in any way in a IWB when it's of the type you describe. When I go from CC to OC (restaurants and last night) I have my .45 Govt in an IWB holster. When dedicated to OC for a day working or relaxing I use a blackhawk CQC serpa. I have not had any problems with either setup.
I have seen threads on several boards discussing OC w/ IWB. A firearm that large where the grip, top of the receiver, and cocked hammer is showing; no one who sees it will doubt it is a sidearm. This might be questionable if you are using a smaller pistol that could easily be covered by a fold of the shirt you are wearing.