I don't personally believe that someone wishing me a Merry Christmas, Happy Hannukah or Happy Kwanzaa translates into them making any assumptions about MY position on faith.
It's only natural to use the holiday greeting that best suits your religion (if it's a religious holiday).
The only thing I could see political correctness being more appropriate is like you already said, if you don't really know the person - say, a stranger on the street. But if it's a friend or acquaintance you're familiar with, using any greeting you're comfortable with should be fine.
And you never
HAVE to be politically correct. As far as I know, they're not arresting people for it (yet).
