Both hands but then again I'm ambidextrous. Before I knew this fact though, I initially started shaving with straights using my dominant (right) hand only. Relegating yourself to just your dominant hand certainly increases the shaving difficulty but not terribly so. Most as you can plainly see eventually overcome this effect.

However, once you get your straight razor, do try using your non-dominant hand on just say the cheek flat. See how it goes. Make an informed decision and go with it!

Either way you are still shaving in the coolest manner possible. YMMV and all that...