A lot of times the horse prefers the rider to post on a certain diagonal , no matter the direction. They will almost "toss you up" with their way of moving that makes one diagonal feel good and the other feel wonky. On the straightaway it doesn't really matter , though its better for the horse to not be allowed to stay as unbalanced in his muscular development. If one side is weaker and he wants you to be always posting off the other one, you should be sure to work him on both his easy AND his hard side.
Cloud HATES me to be on the right diagonal! He will either kind of 'skip' or slow and start again so that I'm on the left. It's pretty annoying :s
"There are two ways to live your life. One is as though nothing is a miracle. The other is as though everything is a miracle." -- Albert Einstein