There is actually quite a variation in the tails seen on hunters, with some riders not trimming the tail at all while others do bang it (albeit leaving it longer than most jumpers would). My personal preference is something in between the two extremes: trimming the tail to a length that suits the horse without leaving it with such a square bottom.
Lengthwise, you should be aiming for the tail to finish no higher than just above the fetlocks. The horse's conformation and the thickness of the tail determine how much longer I will trim it.
I find that a thin tail looks better if it's slightly shorter because increased length will just make it look thinner by increasing the length to width ratio. Fake tails are available to supplement a thin tail and they are legal in the hunter ring. Sometimes the fake tail will help to balance out the look of the horse as some horses look very heavy on the forehand with just a thin tail.
A thick tail tends to look a bit too blunt if it's cut too short, so those ones are best left longer.
When I trim a hunter's tail, I tend not to cut straight across, but rather I make multiple small cuts on an angle to create a slight variation in hair length. This keeps the tail looking more natural than it would if it were banged.