What's even more scary is when the horse is watching where it's feet are going, but the rider is NOT! Now that's something to be worried about. I have a friend that "likes to ride" but she is FAAARRRR from a horse person and recently on a trail ride, nearly got many people hurt because SHE wasn't watching. Not one of the "gazer" horse, as I call them, were any trouble. As previously stated, don't sweat the small stuff that aren't your problem. If you don't want your horse looking away, thats cool, just don't worry about anyone else. Unless it's a safety issue as in the case of my friend... then rip 'em a new one.... =)
As for the spooking issue, I have found that making the horse face his fears and investigate the booger is usually a better tactic than making the horse ignore it and pay attention to you. You create a reactive horse when the horse is not allowed to find it's own level of comfort.
One is closer to God's heart on a horse than anywhere else on earth!