I personally love tricks, on my days off I sit outside and am either trying to teach my (ahem, rather dunce) dog how to do something new, or playing with Indie and getting her to pick up new things. It's a fun hobby, but you just gotta keep in mind horses are a LOT bigger than dogs. As long as you keep that in mind, and know what're doing then I don't see a problem.
Just be prepared for them to be doing it all the time. Teaching to rear would be awesome, and I'd personally love it... but I know Indie has a future with kids, so I won't teach it to her because it wouldn't be fair to expect children to handle that.
But for example:
Last year I taught Indie how to climb onto our porch and she nearly got how to open the screen door (usually the only thing separating the house from outside)... well, let's just say I've walked in to see a horse standing in the living room looking towards the kitchen
So I'm all for trick training, it's fun and potentially innocent; just be prepared for the repercussions.