On the interstate, if someone waves at you with a gun, it is only polite to wave back with yours.
Tell someone that antique chair is fragile before they sit in it, not after they snap the arm off by accident.
During the morning after hangover is not a good time to berate the one with the hangover about his drinking. He will not think loving thoughts about you and will not be paying much attention to the outside world. Your name will begin with b and end with itch.
Being blessed for sneezing is very annoying for some of us who don’t like religion infesting our everyday lives. More annoying than sneezing itself.
A contrasting unwritten rule about saying “bless you” is that it does matter what religion you are, and your religion can affect what you consider to be polite.
This contrasting rule means that you shouldn’t be blessing people who don’t want to be blessed. Don’t bless them for sneezing, or for giving you some award (a la the Academy Awards), or for listening to your commercial (a la PeoplePC), or for reading your begging sign (a la street people ambushing you at stoplights), or anything outside a religious gathering.
There are plenty of ways to be polite without bringing religion into it.
If you just have to say something when somebody sneezes, use a religion-neutral term or phrase:
[ul]
[li]gesundheit (as recommended earlier by Podkayne)[/li][li]salud (Spanish for “health”)[/li][li]You’re so good-looking (as suggested on “Seinfeld”)[/li][li]Point that damn thing in some other direction (for when you’re grumpy and don’t want to deal)[/li][/ul]
It is possible to train people to use religion-neutral phrases. In my experience, it takes an average of 3 requests/reminders per coworker to train each coworker not to say “bless you” when I sneeze. Well worth the effort, considering I sneeze several times a day, and all of those blessings really wear me out. Not to mention get me annoyed at my coworkers for not paying more attention to what they say.