Forward lean is incorporated into boots, bindings and some skis and the theory is that it helps you get pressure on the tip of the ski to turn better. Some combinations of ski boot and binding can really get you forward, so some plates (the Look maxplate and Rossi T plate) are elevated more at the toe than the heel to counteract some of this. Look/Rossi bindings with the Maxplate/T plate keep the boot sole parallel with the surface of the ski.
As for the 'correct' amount of forward lean, anything to do with finding the 'correct' stance angles is just a big can of worms. While some things will obviously hinder skiing there is little or no concensus on the finer points of exactly how you should be set up on your skis. For those who get into it analysing their stance it is often a matter of trial and error and just comes down to what feels good for you.