Here is a link to a similar topic that came up awhile back in the drummer's forum
http://www.musicianforums.com/forums/showthread.php?t=272092
I posted a rather lengthy reply on the topic about halfway in. Check it out. Regarding the hand positions to be avoided question, I would say to avoid playing while lounging on the couch because this really puts a lot of stress on your wrist when it really curls around like that. Try to keep the wrist as straight as possible (this is why playing with your instrument around your knees is a bad idea.)