It's customary to drop the left hand below the grip and just acknowledge an approaching bike. I do it all the time when in Spain.....and to any bike, if they want to reciprocate they will.
Now in UK, i posted awhile ago a (i assume) young un on a f-off sports bike coming towards me in traffic, obviously saw i was on a Goldwing and proceeded to give me the W--nker wave. Nice.
So i chased after him....................oh sorry i went into a dream.
Anyway, i nod all the time, i would say 50% reciprocate. If i was on a sports bike maybe 70% would return the nod.
Am i bothered........let think about that.