Where I live you have to register based on where ( and for how long) the car is garaged (aka parked). If garaged more than 30 days in any location you have to regiser in that location. So if you spend 3 months in one county and 3 months in another and the rest in yet a third county you would have to register in all three. A cop would have to see you often enough to catch on so the risk is small, in most cases. I work next door to the local Police station so no way I can get away with it. Also the DMV wont accept PO box as a registerd address. You also need proof of residence - uitlity bill, mortgage, rental agreement etc...