I tend to wave to anything classic and respond when waved to. One feels a little ridiculous sometimes but like Giles I feel it does not cost anything.
Jay: two of us on Guzzis were belting along at well over the speed limit when a couple of motorcycle cops coming the other way waved. For a moment I thought we were being flagged down but no, they were being more motorcyclists than cops, so we just waved back and kept going. I think the Italian MC cops have a soft spot for Guzzi even though they mainly ride BMW now.