The boat and trailer move in different directions. Your placing stress on the motor lower unit. It wasn't designed for this stress. The transom was designed for streeses placed on it from the motor, even on the highway and back roads to and from the ramp.
As to the original question, 10" is plenty of clearance for the road. It's the steep entrances and exits to businesses you have to be careful about and that depends on the distance from the trailer tires to the skeg.
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