In most of the literature that I have seen, the recommendation is for the left thumb to be roughly in line with the index finger, running parallel to the frets. I find that, especially toward the first fret, this causes excessive wrist bend. If I move my thumb closer toward the floor and angle it so that it is more of a 45 degree angle, I find it helps me keep my wrist straighter. Are there any problems associated with adopting unorthodox thumb positions?
I've realized it's possibly some type of optical illusion. In the first picture I have my thumb low on the neck and the wrist has a slight bend.
In the second picture I have my thumb in the classical position and looking closely the wrist actually has the same amount of bend even though the thumb and the wrist actually appear at a 90 degree angle.