I say Merry Christmas no matter whom I'm saying (or writing) it to. My feeling is that if they are offended by my saying it, then they should be at work when everyone else has the day off for the holiday. You probably won't find that happening!
I wonder if you would say happy birthday to me when it's your birthday and not mine
as far as "you won't find that happening", quite the contrary, where I work, people take off on their own holiday and not on other's holiday, if the owner has his holiday and wants to give everyone the gift of that day off as a bomus of course everyone takes the bonus, but that's the only time anyone takes off on another person's holiday where I work, I am going to bet that's the situation in most work envirnments
this season it's not just christmas, though you apparently think it is, it happens to be just christmas to those who celebrate the holiday and don't understand or even care that there are other celebebration
just for your information, in case you didn't know it yet, it happens to be nother holidays for a good portion the people we meet and greet...just so you know, you are wishing someone happy birthday on a day that is not their birthday
me, if I know what the person celebrates that's what I acknowledge in the greeting, if it's christmas I greet them with that, if it's chanukah that's my greeting, if I don't know what they celebrate then they get a happy holiday
there is no "war on christmas", what there happens to be is a war on everyone who does not celebrate the holiday by people who actually claim saying "happy holiday" is somehow a bad thing
as I said, why would I wish someone else happy birthday on my birthday, that would be really rediculous and I feel the same about wishing someone merry christmas even though it might not be their holiday
sazar has the best solution in the post directly above mine