I think this is probably one of those topics where the 'line' is going to be different for different people.
For me, the OOC aspect to corp-theft as a RPer is a bit

Unless you have absolutely
no OOC contact with your corpmates at all, at least some of the trust you're building up from them is going to be OOC as well as IC: to get to a position where you can steal from them, you're going to have to lie to them, by omission at least. The deception is not just by your character, but by you, the player.
Personally, I don't think I could do that, although I know others do, and it's part of the rich tapestry of Eve.