Most of the etiquette for Rummy is built into the rules. For example, it is etiquette to "call" a Joker as both a card and a suit the instant it is placed on the table within a combo. That said, tournament rules do add a bit of game play to them that you often find implemented in various groups to some extent. I've also been at a new table (mostly with relatives it seems) before and been surprised to see play accepted that is widely considered "coffee-housing."
My favorite on-line help page to point new players at is: Rummy Talk.