In my opinion these clerks, etc, are just doing their job - trying to make the multitude happy.
To me you have four choices:
- If you do not celebrate, just say thank you and basically ignore them but be courteous about it.
- If you celebrate in the same religion, say "Merry Christmas" back.
- If you do celebrate in another religion, reply with your religion's response. I doubt if they will care.
- Say nothing. Why ruin the clerks, etc day?
Lighten up. I do not think it matters.
Z