Well all these responses sure make me feel a heck of a lot better!
I have NEVER focused on diagonals. Riding a western horse at a jog in the show ring ... well, you don't have to! Out for general riding, I do post when we are at a good working trot but 99% of the time I am on the right diagonal. It is just comfortable for this.
This fall, I started trying to force myself and FEEL which one I was on. Plus, I figure it has got to be annoying for my horse if I am always riding on the same diagonal. For his physical health, I really should switch it up. So I've been working on it, but I do find myself glancing down a lot to check!
Diagonals, I think, are much harder to feel than what lead your horse is on when loping. I never have to look down for the lope.
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.