I completely agree with everything Chopsticks said.
My guy is coming three in May and is a Percheron/paint cross. He hasn't been level for more than a month or two that I've had him, but it's just him growing. That's why, when someone asks for a critique on a young horse, I NEVER mention that they're butt high. It's usually rather obvious and they'll usually grow out of it, even if they're REALLY butt high.