I believe most Japanese think of Christmas as a secular, commercial holiday (gift giving, christmas decorations, etc) rather than a religious celebration of the birth of Christ, so I would think that most would not even think to be offended. It might be out of place to say at a religious (Shinto or Buddhist) shrine or celebration, but I would think this would be obvious. The only reason it became politically correct to say Happy Holidays in the U.S., for instance, is due to the increased presence of religious minorities or atheists in what had been a firmly majority Christian nation. As Japan is one of the most secular nations on earth (with a Christian population of less than 2% of the general populace), I would think this consideration to be mostly irrelevant.