I have a 2010 XF with 19 inch rims. I see that factory has a 49 mm offset. Does my new rims have to be at 49 mm offset as well or can I go with different sizes that are somewhat close?

 

What other offsets are you considering? You want to go wider?

 

I'm still looking at the 19x8.5. The offset on the rims I want are 44 mm instead of 49 mm. I've also seen some that were mid 30 mm that I like.

 

You should be OK with 44mm - mind 30s might be OK as well, depending on the tire you plan to run - how wide is going to be and the type of the thread.
Also, make sure that the side wall on the tire is not too tall - keep it at most stock height (which I think is 35!?!?).