Now to the original post. NC law does not require you to be completely concealed with a permit. The permitsimply gives you permission *shudder* to carry concealed if you so choose. Since you have a permit, under normal circumstances you can carry however you want. IWB, OWB, OTOH (on top of head), it doesn't matter.
For those that do not have a permit, IWB holsters are risky. Are they concealed? Well, you can still very easily see that you have a gun, so probably not. Is it VERY easy to accidentally
conceal the weapon with the fold of your shirt and such when carrying like that? Very VERY easy. Could an IWB holster be misconstrude as an attempt to conceal? Probably so.
Legality aside, I wouldn't suggest using an IWB holster w/o a permit. As far as I know, though, it's not illegal, either. Keep in mind, I'm not a lawyer.