I had a pretty significant row with my oldest sister after she found out that I not only wasn't a Catholic (the religion of our childhood and hers still), but I wasn't a believer at all anymore.
She seemed very threatened and was actually rather insulting. She assumed many, many things about my worldview without asking me a single thing about it. I don't think she has ever specifically known many Atheists or Agnostics, despite her being a professional woman living in a large city, so when she found out that her sister was one, well, I'm sure that was confusing and scary.
I have lived in some pretty liberal places and have worked in at least one business where diversity and difference were very much embraced. I've had great discussions with coworkers all along the religious spectrum, although mostly I've encountered Christians of all sorts, Buddhists, Jews, Wiccans, Deists, and non-believers. Buth my workplace was unique, I am afraid.
In general, I don't mention my Agnosticism unless I have some sign from my audience that it is OK to do so. I have no interest in making others uncomfortable or arguing with then nor do I particularly want to become the object of an conversion attempt.
"Science is like a blabbermouth who ruins a movie by telling you how it ends! Well I say there are some things we don't want to know! Important things!"
- Ned Flanders
"Question with boldness even the existence of God; because, if there be one, he must more approve of the homage of reason than that of blindfolded fear." - Thomas Jefferson