You know, I think it's done all the time but I always thought that it was a courtesy to the riders not to stand at the rail. The horses SHOULD be used to it, but in fact there is always the chance that one fo the horses ISN'T used to it.
If you do stand at the rail, it should be a place where the horses can easily see you on the approach, from a distance, not just when they get to you, so that they don't spook. And of course people shouldn't be moving around or taking pictures, which I see all the time. The trainers (or moms) get on the rail and yell instructions to the riders, too - so if they can do that it would seem that other quiet people should be tolerated on the rail, right?