The main thing with equitation is having a nice full tail not so much cut in a straight line. As long as you can make it look like it's full and as long as it's long you can touch the end of it and cut just the ends off to give that finished look. If your horse's tail is not anywhere near as full as it should be or even have the length, I would just make sure its clean and as full as it can look without cutting the end.
If you're going to a recognized show (Paint, Pinto, Quarter horse, etc.,) You have to cut it. Have to, or you won't even get LOOKED at, lol.
If you're just going to 4-H shows where western pleasure may not be that competitive, you can band a long, neatly kept mane. (Some reining horses have their long manes banded... looks pretty!)