My biggest problem with barr chords and power chords is my left thumb. I never thought about it before because I can barr fine (so I thought) but recently noticed that my thumb sticks out to the side parallel to the middle of the neck neck instead of in line with my first or middle finger like you are supposed to. It points toward the headstock if you see what I mean. I'm working on correcting that but its throwing my fretting fingering positions off a little as I retrain my thumb to lose the bad habit! When I do open chords and solo lines it is always in the right position.

This is a natural position of thumb - and actually the most comfortable for playing.
Glad if i answered the question!

As Pavel states thats the correct positioning.. However, sometimes while holding like that I experience discomfort so hence I sort of developed another way for my thumb. The position Pavel showed DOES allow you for the best access to the fretboard but if you, like me, experience any discomfort try to change the position of the thumb. By this I mean dont stay static, just move your thumb around a bit if you feel the slighest strain.

Hope I helped, if not I blame any ramble on me being too tired to move the 1½ meter it takes to get to my bed.

Thanks for both your suggestions. My thumbs are kind of double-jointed so maybe that's part of it. My thumb supports my hand from the centerline of the neck, just 90 degrees from where it should be. I've been consciously trying to force it to the correct place lately. Sometimes it just pops over. I'm thinking about trying to wear a thick rubber band around my hand to help get my thumb used to it. Funny my oldest daughter is double-jointed too and plays her violin the same way! Guess its a genetic thing that runs in the family