well..this has some kind of history back in the 50th, 60th *as ive been told by my Dad cause there were only a few bikes on the roads like the Horex etc* and 70th and we kept doing this as a ritual up to the 90th, talking about the ol days back in Germany...
anyway...today...who gives a flying fark to wave at somebody you dont know and taking your hands off the handlebars...here in DR everybody you know personally will wave at ya or just shake the head as a sign of knowledge, without any importance the brand you have, and thats cool...
with this modern bikes and powers and traffic its just better to keep the sticky side down with both hands were they belong to be..just to be prepared in case some idiot in front of you braking hard or for whatever reason, you name it, to be better prepared for action, reaction or reflexes you might need in a specific moment though...