Hi pconover,

Form my experience, it's not a big deal if you hang your thumb over the fretboard when playing some chords. As long as you use your thumb correctly when you need to, you should be fine. For example, when I play an A major or a D major open chord, I tend to rest my thumb near or around the the sixth string. This is alright because it doesn't change the sound of the open chords I'm playing.

But!

If I'm playing a G major barre chord on the third fret, or say, a B minor chord, I'm definitely going to place my thumb behind the fretboard, on the neck to help stabilize my hand. I think with practice, you'll simply gain some muscle memory and your thumb will be in the correct position without even thinking. Granted, this does take a bit of time, practice and patience.

To answer your question: it's nothing to worry about.

Best,

Billy