“Aunt Flappy’s Guide to Pilot Etiquette” prescribes:

Any pilot — head nod
Same company — single wave
Same aircraft type — double wave
Same seat position — two hand wave & one foot waggle
New-hire classmate — assume “deceased insect” posture and flail with all four limbs while emitting loud squeaks.

A less-than-correct wave is a faux pas, while overwaving is considered obsequious.