Left so you can control the crown with your right hand, I'm guessing your right handed?
I've always been told to wear a watch on my left wrist so that's what I've always done.
OMG, I never thought about that problem, looks like Apple dropped the ball on that one,
instead of an extra band they should have added and extra watch !
The only solution would be to print an exact copy of the watch in paper and strap it to the other arm, to lessen the shock of raising the wrong hand, pulling back your sleeve and seeing....... no Apple Watch,
and if you are really brave write "Look at other arm" next to it !
I do hope there will be an Ambidextrous support group soon !