Yeah, tail length doesn't matter. When they're running, their tails are too far behind them for them to step on it. Some racing trainers bang their horse's tails since it looks nicer, but otherwise I don't think anybody really worries about how long they are.
Im assuming you are talking about thoroughbreds correct? We don't trim our standardbreds at all. Most of them don't get to be long enough that it requires cutting to begin with. Although I believe with thoroughbreds its popular to trim their tails to about the level of their hock. When I bought my thoroughbred his tail was cut to his hock, also his mane was pulled.