If you've had no problems whatsoever in playing your LH with a high wrist and flat knuckles, and it hasn't bought you any tension or limits of any kind, then I'd say to stick with it. Sometimes being conservative is a good thing.
However, if you find any limitations from playing your LH with a high wrist and flat knuckles that you think you'll be able to overcome with a low wrist and bent knuckles, practice a run slowly making sure you completely relax between each note and take as much time as you need to relax. Eventually you'll be able to play that run fast in your new wrist and knuckle positions without any tension problems. Also talking to your teacher about it will be good. But as far as I see it, if playing with a high wrist and flat knuckles isn't limiting your playing whatsoever, keep with it despite what your teacher might say. The biology of your left arm/hand might just be different to your right and other people's left.
Henrah