If I'm playing barre chords, or planning to play barre chords soon, the thumb's round the back. If I'm playing an F and using my thumb to fret the bass string, it's round the side. Rest of the time it's somewhere in between. Stick in one position and I get cramp anyway.In fact, when I'm not playing barre chords, most of the pressure, for me, doesn't come from the thumb at all, but from my body pressing on the guitar, and the fingwers pressing down on the strings.
But the size of your hands and the size of the instrument neck also comes into it - classical guitars tend to be built in such a way you really have to do it the official way.