They are class legal, but hardly anyone uses them. Of the leech tell tales I've see used by folks that fininsh well is one near the top batten, (and that is RARE) or one near the top of the sail placed inboard by about 6-8" from the leech.
I have found over the years that leech tell tales are really not that important. Generally if you have the normal luff tell tale setup and you get them all flying correctly the leech follows suit.
However on the 8.1 rig leech tell tales are a must because of the way the sail is cut. I would be lost without them!