Watch the Hand Position online guitar lesson by David Grissom from Open Road Guitar
It is important for me to be in control of the guitar as opposed to the guitar being in control of me. This starts with hand position. I start with my thumb over the top of the neck, this puts me in a position of strength over my bends, controls my vibrato, and gives me an extra finger to fret with. Some people don't take advantage of this and to me that's like leaving money on the table! I am constantly fretting bass notes with my thumb, it allows me to play pedal tones below notes, play chords I could not play otherwise, and more. You think about scales, chords, or anything else, you really need to consider how you hold the guitar.