Depends on your build. who trains you, and who you listen too.
I prefer to start just behind my toes and pull in towards my center of gravity.
A good level of co-ordination will enable you to pull cleanly with the bar starting touching your skin.
many people however use to much energy, and lose to much power and direction trying to avoid the knees.
This is also a timing problem...............getting your knees out of the way before the bar gets there.
I dont like this postion how ever. Due to the length of my arms if i didnt start further out, by the time i got to my knees (with them straight and out of the way, my back would be at a undesirable angle.