I ride this monster

and I wave to EVERYONE. People on metric bikes, people on american bikes, people on scooters....heck, I'll even wave to bicyclists sometimes. I figure if you're out on two wheels then good on ya.
I have found, though, that many Harley owners don't wave back. Maybe they think they're too good for me, or maybe they bought the bike to prove something and they really aren't riders so they are terrified to let go of the handle bars long enough to give a quick wave back.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless