I've noticed that I have this problem with my left hand's thumb. Since I'm a righty, I'm using my left hand to fret notes, with my thumb on the back of the neck of the guitar, at about the center, vertically, of the neck. However, I've noticed that when playing on the higher frets (about 10-12th up) for an extended amount of time, my thumb turns and the side of it starts pressing hard into the back of the neck, with no conscious decision to move from me. This is causing a lot of tension in my fretting hand, and I'm not sure what to do about it. Does anyone know how to eliminate this?
You have to pay more attention to when and why this happens - there is a reason, it's probably you changing posture when you bend, at a guess. Then, when you find out what the problem is, you focus while playing slowly and avoiding the mistake in future.
This has to do somewhat with placement of the fretting hand thumb. I know there are no absolutes when it comes to electric guitar as with classic guitar, but what is optimal placement of the thumb when it comes to lead playing. I was practicing YYZ's main riff, and started off with my thumb muting the low-e (I'm accustomed to this from playing chords that require it), but as I practiced more, I felt it was comfortable to keep the thumb pointed towards the guitar head (sideways) on the middle of my neck. I felt it gave me more control?

So my question is, am I practicing the correct things? Should I modify how I place my thumb, or is the way I described it suitable?