I played steel-string guitar for a couple of years, then switched to a classic and started taking lessons. I keep the ball of my thumb on the midline of the back of the neck. The whole idea is to position the hand so you arch the fingers over the strings and not, say, dampen the second string while you're fretting the third. I've been using essentially classic technique since the mid-Fifties and I can't say that I've ever had occasion to fret a string with my thumb. With the thumb where it is, full bar chords don't present a problem.

What causes problems with the left wrist when using this position is the overall position of the guitar. If you hold the guitar with the neck angled up about 30 to 45 degrees, the wrist should be comfortable. If you angle the neck down (typical of rock guitarists and some C & W guitarists), it may look "cool" (definition of cool: not so hot), but you're flirting with carpal tunnel syndrome, even with the thumb wrapped around in front.

But -- if I have occasion to need my thumb to fret a string, I'll use it. Whatever works. But then I'd return my thumb to the back of the neck, because it's ergonomically sound, and for me it's just more comfortable.

Don Firth