Paul,
like a bunker in the middle of the fairway, the long rough and trees were there to see before you hit the shot so they are not really unfair. On the other hand where does it say in the rules golf should be fair so purely on this basis no it shouldn't be cut shorter.
However, Joe hits the nail on the head. It all depends on the type of course, standard of player and situation of the hole. If these suggest it should be shorter then get cutting.