I say "Salud!" (health in Spanish). At least it doesn't seem superstitious. It actually seems relevant. Given the fact that I'm around Spanish-speakers all of the time it is more acceptable.
I think it is a silly tradition. When someone's allergies are going crazy I wish I could just leave them in peace, but I don't want to be labeled anti-social. Indeed, if I am sneezing, I prefer to be left alone and not have to return a "thank you". If I am friendly enough with certain people who feel as I do, we just agree not to bother each other.
Belches, farts, yawns, and hiccups are also involuntary outbursts. Why don't we "bless people for that". Well, okay, sometimes I do bless myself for a good belch.
