I would prefer that they make it do the same damage no matter where that you are standing in relation to other people and / or the monster but, give it an additional effect when done from the ideal position.
IE when sneak attack is landed from behind, the monster gets hit with a 3% crit rate increase for 30 seconds.
or
When Trick attack is landed from an an ideal position, the thief gains increased enmity decay or just loses a chunk of enmity (maybe half?).
