"Coming out" to my friends was sort of funny because most of them were either poly themselves or poly-friendly. So it was a kind of "hey ya'll, guess what I'm actively doing now too?"
(and then I ended up dating a couple of them. of course

)
At work, I'm not out to everybody - but anybody who sits near me or knows me well enough for me to consider them a friend knows that I'm poly. If I were point-blank asked, I would confirm I was poly and hope I didn't just kill my career - I don't think it would. I live in a pretty progressive area.
For family, I came out to my mom first a couple of years ago. I don't live near my family, and we're not particularly close - I go home for one holiday once every couple of years. I didn't tell them about me being poly at first because I really don't tell them much of import about myself ever. But, after I met Jon, and realized that we were going to want to live together and that that meant living with Lora, I knew I had to tell my mom.
Before I actually told her, I semi-outed myself (or at least gave her something to question). We were out to brunch (she visits me twice a year), and she said something about how Jon and I were getting serious, and while she liked him, she also really liked Dan (a guy I'd been seeing more I met Jonathan, and was still seeing). I said the first thing that came to mind, which was "oh, I'm still seeing Dan...", which raised an eyebrow. I told her I'd tell her about it more later (didn't want to have the conversation in a restaurant). So I told her the full story that night.
She took it well, in that she was pretty resigned by now to the concept that I was going to do whatever I wanted, regardless of her feelings (it only took 10+ years, but she's now aware that my life choices are not All About Her). She asked me the following things: 1) Don't tell dad 2) Don't be out on Facebook (where I'm friends with many family members).
I told her Dad needs to know, because he comes to visit me, and since Jon and Lora and I were all going to move in together, we decided that anybody who came over needed to know that we're poly (no pretending that Jon was "just roommates" with one of us to keep people ignorant). She argued (and I really wanted to smack her) that it really wasn't *that* important...it's not like we were all going to have sex in front of Dad or something, right?
(let's just take a moment to all sit with the thought that my mom came up with an idea like that. yeah. she did. and then she said it
out loud. to
me.)
So I said first of all, we don't all have sex together. Secondly WHAT IS WRONG WITH YOU?!?!?! Thirdly, if, as you put it, it's really not *that* important, then telling Dad isn't *that* big a deal, is it? And I told her that I would be telling him before he came to visit, or she could. She decided to tell him. The heavens didn't open up, nothing bad came of it, as far as I can tell he blinked and said "ok" (my Dad is a rather laid back person; I'm not sure what the problem with telling him was, other than life-long societal censure about non-traditional relationships) and moved on.
For the facebook thing, I told her I would carry on the same as I always do: I don't put my relationship status on FB, I don't write long, emotional posts about people I love, but I do put photos up of me and the people I'm dating (or other people do, and tag me in them), while cuddling, kissing, hanging out. And I do sometimes mention them in ways that a person could discern mean that we're more than "just friends". And sometimes they post things on my wall that are clearly more than "just friends" sorts of things.
So anybody with a brain who looked at my photos or read my wall would probably either know that I'm poly, or have some idea that something other than plain monogamy is going on.
Anyways, for me, coming out wasn't/isn't about the stability of my relationship(s). It was about wanting to live authentically and not worry about tiptoeing around not talking about certain people or having to do a huge amount of internal monitoring to remember what was OK to say and what would be outing myself. Much like monogamous people, sometimes I talk about my significant other(s)/lovers/people I'm dating at work/with family/whenever. I'm not going to share my sexlife, but I might talk about how we went to a great B&B over the weekend. Or talk about how my metamour got a great job offer and I'm really glad for her. Or whatever, the kind of chit-chat people make when they want to say something a little more descriptive than "it was fine" when asked about their weekend.