In the classical position it is very difficult if not impossible to use your left thumb (for right handed players) to deaden or play the low E string. Using your thumb is a must for rock guitar. The most famous "thumb use" is probably Hendrix, who could stretch over several strings, but the technique is used in many of the "classic" rock riffs we all know - Jimmy Page would be another example of lots of thumb use.
My point is that the "best" position depends on what style you are playing. For me, getting my thumb more involved has been a quest since I first learned the riff to "Whole Lotta Love".
walrus