It happens, it depends, not sure also how well it is enforced either.
I can admit I am guilty of this with one of my chars actually this char has her fingers in three factions. But through and through her RP is still fully based on the ID and IFF being used. I don't change that in my RP, in fact it is essential for her to keep acting as her ID and IFF and seemingly be absolutely no different then her peers around her.