you should wear your watch on the opposite hand you hold a cup/glass in, this way when holding a cup of beer if someone asks you what time it is you won't spill it all over the floor.
you are supposed to wear it on the opposite hand you do shit with. However, it really doesn't matter. Wearing on the opposite hand just keeps it out of the way when you write, eat, wipe your ass, beat off, whatever.
doesnt matter if you are asking about customs or formalities..... wearing it on your non-dominant hand makes more sense, but only for small matters such as not weighing down your dominant hand or getting in the way or just for being able to adjust your watch easier with your dominant hand.
Traditionally you wear your watch on your non dominant side, leaving your dominant hand free to wind the watch. Nowadays most watches are kinetic so I guess it doesnt really matter.