I have taught my horse several tricks (fetch, high five and shake hands) and she will do these all the time, even when I don't want her to. This isn't really a problem because its kind of cute and it doesn't cause any problems (except you can't leave any saddleblankets on the groud coz she picks them up!). Before teaching your horse to sit, REALLY think about whether its something you want him to do all the time and he could do it anytime. Knowing horses, he will do this at the worse possible times! E.g. At a show, in a lesson.... I know its really cool having horses that do tricks, but perhaps stick with somthing that won't be too annoying if he decides to do it all the time!
High 5 with their nose is a good trick to teach because it can't really be too annoying if they do it heaps. Shaking hands is also good because it means that whenever you reach for their hoof, even to pick it out, they will lift it for you!
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare