I don't know that people would use 'azz' because they thought 'ass' was offensive. Offence, I'd have thought, is the last thing that the typical texter would be thinking of! I don't use 'ass' in that context because the Australian word is 'arse' (which, incidentally, rhymes with the way I say 'pass'), nothing to do with being offensive. I, too, don't think that ass/arse is offensive any more. [in spoken language, I'd put ass/arse down to accent rather than different words and I'd have thought nothing of just saying it the way I do, on paper it sort of becomes a decision. So if there were a time when I would have said 'ass' as if I were switching to an American accent, I'd write 'ass'.]