There is no such thing as bad technique.
Everybody's hands and fingers are different so what works for other people might not work for you.
Hell, if I paid attention to half the guys who told me I was using the wrong techniques, I wouldn't be able to play half the stuff I play now.
And that includes where the thumb goes... move it to where it works best for you depending on the chords, the style of music, your finger length and size.