Always line it up to the rear, as the front may well be towing correctly but doesn't mean the rear is following the front, the car will be trying to drive at a slight diagonal, If they lock the wheel with the tool, set the correct toe, and line up to the rear, there is no reason why it wont drive in a straight line, worse case, steering wheel needs to come off to get it straight, but usually only if its been off before and messed up.