Of course if one knows that someone celebrates something else it is wise and kind to speak to their own particular holiday. But when one does not know, a generic "merry Christmas" is always fine. It is a national day off and so it has become a secular holiday instead of a religious holy day for many.
Although I don't support the idea of America being a Christian nation, Merry Christmas is all but completely commercialized as a phrase at this point. It's a mostly irrelevant argument, although I can see the point from where people in other religions, who do not celebrate the holiday, might balk, it is, in America, only a nominally religious holiday.