Yes on every instrument I play (violin, piano, guitar and voice). I learnt piano and violin early on, and the theory I learnt then makes me a much better guitarist. I can work out chords from first principles, where some other guys I know at folk clubs (who are otherwise pretty good guitarists) need to have them written down for them.

Some things, like familarity with the fingerboard, you just have to learn from doing it. But there's a lot of stuff which teachers can tell you about how to play better. On guitar for instance, I've learnt that keeping my left-hand elbow close to my side (rather than stuck out like a chicken flapping) gives my fingers more range of frets. You will *never* correct a bad habit on your own, bcos you won't recognise it as a bad habit.

Graham.