One thing to note also is that the water being pushed along the side of the boat has an influence on the direction. If you're not in the middle then your bows will be pushed away from the nearest bank (This phenomenon can be used to push the bows out from the bank when starting of in a forwards direction, a blast of reverse will set the bows moving away from the bank. I still find it easier to walk 50ft, push with one finger and then walk all the way back).
The shape of a boat affects how it will steer, both forwards and in reverse. The more 'pointy' the underwater profile to more it likes to travel in straight lines.
I once saw a working boat reverse from Stoke Bruerne top lock to the winding hole by the tunnel. It's narrow, with moored boats on one side and round a bend. When I congratulated the steerer on his skill he shrugged and said: "It steers in reverse this one."