Let's put to one side the merits of wishing someone a Merry Christmas, or for that matter of getting offended if someone wishes you a Merry Christmas. Suffice it to say that some people feel that getting offended at "Merry Christmas" is overly touchy...even provocative.
Self-defense expert Marc "Animal" MacYoung comments: "If they're looking to pick a fight, can I punch them? Oh wait, their version of a 'fight'"
Different people have different versions of what's a fight and what's violence, based on their views of what's acceptable and unacceptable. One person has no problem with a verbal conflict, but draws the line at touching much less hitting.
The thing is, not everyone draws the line that way. The law itself, in certain places (for example, Virginia) rejects the idea that "You can talk but you can't touch." That's because with many people - especially (but not only) in certain regions, neighborhoods and socio-economic groups - talking can lead to touching, slapping and all-out beating. You and I may see the latter as an unjustified attack; many other people see it as a simple escalation - along a single continuum.
Bottom line: You may be able to start anything you see fit by your rules. However, the person you start it with may be able to finish it by theirs - and you may have bitten off more than you can chew. (Assuming you can still chew, that is.) And even if things don't get physical, we need to understand that the other person may be willing to take things much further than we've ever dreamed. For example, what you consider to be simply a direct and forthright response to a co-worker's unfair request could result in her plotting to get you nasty assignments...or even fired.
One of the easiest and worst mistakes in the world to make is about (1) how someone else perceives what you do and say and (2) what they're capable of.