The tail doesn't need to be cut to the hocks - this is a big mistake that a lot of beginners make, they see hacks going around with their tails at the hock, take a pair of scissors to their own horse's tail and cut it to the hock.... they fail to take into account the fact that a horse will carry it's tail while working - so if you cut it to the hock it'll end up way above the hock when the horse starts moving!!
I do think tails look much better, neater and fuller when squared off, being a dressage rider I like my horses' tails to be squared about 1-2inches above the fetlock, and leave the top of the tail natural.
As for manes, if you are needing to plait to show, the mane will need to be pulled or your plaits will turn into big chunky knobs on your horse's neck that will not do it any favours at all.
But please don't cut it! Get someone experienced to show you how to pull a mane. Pulling it just takes some hairs from the bottom of the mane to thin and shorten it, making plaiting much easier and the end result far neater.