Virtually all of nature's creatures have lateral symmetry in respect to interaction with their environment.
Dave, don't mean to put the dampener on what is turning into another interesting thread, but most folk are not ambidextrous...
IFMU, thanks for pointing that out - my goof. I imagine indexing rotors to cross at +/- 22.5 deg to the 90 deg azimuth points reduces vibration from both blade slap above structure, and having downwash from two blades impinge structure simultaneously.
Mart