Working as a cashier at a major Canadian retail store, the unspoken rule last year (my first Christmas there) was to say "Happy Holidays".
Usually I would say Happy Holidays, and if the customer caught on and replied with "Merry Christmas", I would reply "Merry Christmas". That way, no harm, no foul
I think, with my store at least, when the music has already started being played before Halloweeen, Christmas trees up, walls of Christmas lights, I think its a pretty tough sell on not saying "Merry Christmas".
I celebrate Christmas as a time to get together with family and friends. Its my absolute favourite time of the year.
That said as a Rastafarian, I have two spiritual Christmases a year.
One Love
Ras Marques B.