Heish, I struggle to find a comfortable posture on either classical guitar (nylon), bass or electric guitar.
I also default to classical position (seated w/guitar on left leg with left foot raised on stool, neck angled upwards), my left arm is quite comfy here, left thumb solid behind the fretboard & parallel to the frets - but find that the angle of the right hand doesn't get the same tone if I sit with guitar on right leg, neck parallel to ground (or just pointing slightly upwards - I call it cowboy style :?) - this was pointed out by the flamenco teacher, but after some trial and error, I found my left arm (shoulder) takes strain and the left thumb tends away from parallel when playing 'cowboy style'.
We've debated it - he has also had one or two issues with neck/shoulders - the sum total of our debates has been : For flamenco - The typical spainish body type is a shorter human with short arms - making a 'cowboy' style position easier for that body type on a typical classical guitar, with the right hand (and wrist) working well between bridge and soundhole. I'm also shorter (5ft6in) but have orangutan arms (long ?) so I find that my right hand is between the soundhole and the neck - so I lose out on some flamenco tone. When I compensate to get the right hand striking the strings below the soundhole then the guitar neck tends to get pushed away and the left hand has to reach for the neck - less than ideal.
I typically play electric guitar/bass cowboy style (seated), it's easier because I find I don't have to reach around an acoustic body - standing I play the guitar/bass quite high up on a strap, neck angled upwards - looks a bit uncool, feels okay... ???
The healthy guitarist (google books preview)
http://books.google.co.za/books?id=hWcAAuOUlUgC&printsec=frontcover#v=onepage&q&f=false
I've been meaning to give this a read sometime, but not quite got around to it yet, perhaps too technical for me... - similar to a quesiton Alex was asking a few weeks ago - regarding posture while playing.