All notes must be fretted with the tips of the fingers and positioned as close as practical to the fretwires.
There are basically two positions for the left hand. In most cases the left hand thumb should be positioned behind the neck of the guitar with the fingers evenly arched over the fretboard.
When techniques such as the bend, release bend, vibrato etc. you may find it more comfortable to have the left hand thumb in a higher position, wrapped over the top of the fretboard.