Dear David,
This question always seems to come up while teaching Heidelberg Catechism #99 re the Third Commandment, specifically:
In a word, we must not use the holy name of God except with fear and reverence so that he may be rightly confessed and addressed by us, and be glorified in all our words and works.
(or as my pastor paraphrases, "use God's name only when speaking
to Him or
about Him)
I normally say that if, when someone sneezes, our thought, even briefly, is running along the lines of:
"wow--he might be dangerously sick--I should pray for him--Sovereign Lord, Creator of heaven and earth, apart from whose will not a sparrow can fall to the ground, if it be according to your good pleasure, bless him with healing and restoration and preservation (and if an unbeliever: to lead him to saving faith), for the praise of your glorious grace in Christ Jesus"
then, by all means, feel free to say "God bless you".
But if we are about to speak the words automatically,
"just because that's what you're supposed to say when someone sneezes"
then it would be better to say something like:
"that's some cold you've got!" or
"want a tissue?" or
"please cover your mouth next time!"
