I tell people, in general, that I have been married for a long time and have an open marriage. If people ask more questions, I tell them about my husband's long term girlfriend or about my recent lovers.
I think you are right on the money as far as feeling out who you can tell and who might react badly. I don't encourage being in the closet, but I also know that disclosing takes some skill and timing. about 95% of the people in my life know. There is one close friend who is an exception. I don't know about work because I am a writer (at home). Everyone I have told has been at worst, disinterested. Ironically, the only person to give me hell is a guy who is gay and "believes in monogamy" but cheats all the time.
When I was first thinking of being poly, I read a great book by the poet Julianna Spahr called (i think) Tranformations. Julianna has had two husbands for many years (and they now have a child). The book (which is poetic prose) REALLY outlines the feeling of being "three" and housing, job events and so on.