An addition: It is important to keep the elbow out, the nut level with the shoulder, and the forearm at right angles to the frets when learning, whether you stand or sit. Nic Jones did tend to sit when playing, as do John James, Dave Evans, Doc Watson, David Lindley, Stefan Grossman and Leo Kottke, to name a few. It's easier to play the hard stuff sitting down, but you can play it standing up with practice. The thing is to learn whilst in the correct position for easy fingering. You can end up in all sorts of positions after you've learned to play, but it is easier to learn if you adopt a good position to start with. It is very easy to say that certain great guitarists learned to play with the guitar in a strange position (Django was a little odd, but if you want UUUUUUH watch a film of Wes Montgomery), but they got good DESPITE the handicap of a bad position.