Always acknowledge another rider with a nod, wave, hello, hand flick as it simply seems the polite thing to do.
NEVER SHAVED until some guy took me out during a USCF Race in Central Park, NYC many years ago. Went down hard and slid, got up and rode from the scene on the West Side to my car on the East Side and then drove back to NJ. Got home and into a hot shower and allowed the dried blood/hair/road grim to moisten then scrubbed the crap out. Any race after that was preceded with shaving BUT only for racing.