What do you say when someone says "god bless you" to you when you sneeze? It seems rather petty to say, no thanks or I don't believe in god but saying thank you kind of annoys me because I hate to acquiesce to their superstitions. When someone else sneezes, I just don't say anything. Its a stupid tradition. You don't go saying something when someone coughs or farts.
I say "bless you".
I also say "oh my god" when taken aback and a lot of "oh god" during sex. To me these are just expressions saying which does not make my Atheism any weaker. I don't get annoyed when someone uses such expressions either. But say "I'll pray for you" and I'm ready to rip you in pieces lol
I've never had a problem with "Bless You". Most people have already dropped the god from in front of it and in no way is it an affirmation of faith of any kind. At least not today.
I can also tolerate most faith based expressions when the're used (Oh god, OMG, good heavens), and actually like a few of them (OMFG, good hell, holy shit, etc...). I'm not even bothered when people pray for me, (it should demonstrate how futile it really is).
As for what does offend me:-
god knows best,
god will provide,
the islamic "insha'allah"
The words on their own are meaningless. It is more the context in how they are used. When one fails at a task or has a shortcoming it is too easy to say...god will provide, or it was not in god's plan to do so. I cannot express how much that angers me!