Everything is a calculated risk. If you're taxiing straight ahead for a while, having them heads down is almost negligible on the risk scale. The flight manuals make this point about appropriate or inappropriate times to be heads down. Driving a car with your hands always at 10 and 2 is also the safest course of action, but I guarantee you don't drive that way all the time. The fo's should be used to a single engine taxi. If they are not, then you are part of the problem. I'm not saying a single engine taxi is always appropriate, but I'm sure there is at least one leg per trip where it would be.