Officially you do need carnet for Iran, I've also heard of people doing it without, but might involve much more paperwork, time and patience, and possibly money as well.
We went from Dogubayazit (Gurbulak) to Maku (Bazargan) and surely they asked for the carnet straight away, seemed to me its pretty much required, wouldnt like to be trying to get there without one. And if they'll make you pay a deposit at the border, do you have any guarantee you will ever get anything back? Much rather have such stuff sorted in my home country.
Especially if continuing to Pakistan and India, it will be essential. In Finland the carnet for our bike was about 250 euros for all issuing costs, and 5000 euros refundable deposit.