I have just been told that the proper way to play is to keep your thumb under the fretboard so that you can keep good pressure on the frets...and I can see the point. I think it also helps you play chords more cleanly. For me, it's most comfortable to have my thumb more around the fretboard. It's not a big deal but I have been trying to be better at barre chords - i am still terrible.

thanks for the repy