Originally Posted by bsms
If the horse is going straight, diagonals don't matter
Not so. Constantly posting on the same diagonal on the straight (like a trail ride) will make the horse sore.
Even when posting on long straights (like across a huge corn or bean field), I will switch diagonals every so often so that the horse gets worked equally on both sides.
Remember that the horse has to push off with your weight on his back every other stride. If that heavy stride is always on the same pair of legs, he's not being worked evenly.