How I was taught, and what I've liked, was that for faster passages that lack any sort of 'blues' touch, you place your thumb perpendicular to the neck aligned with the middle finger. For more bluesy licks, you place your thumb up over the neck to give leverage for those sick bends and whatnot. And for chords, of course it's just 'whatever you feel comfortable with.'
But seriously, there are so many schools of thought on proper left hand technique. Look at your favorite shredders---half the time they have technique that would make most 'proper' players puke! Yet, they get the job done. I've even seen Yngwie Malmsteen use odd left hand technique on occassion.
So just think about what you're trying to play and say 'hmmm...for this insanely fast widdley woo should I put my thumb over the neck, behind it, or whatever?' And you'll just do what facilitates the music--which is really the best thing--IMO.
Back In Black isn't a song. It's a divine call that gets channeled through five righteous dudes every thousand years or so. That's why dragons and sea monsters don't exist anymore.
