Depends on the bottle. I suppose I would break it down into three categories of wine:
1) House wines - wines that I like and I feel to be varietally correct and/or good, and are generally under $15. These I am happy to share with anybody, or just open for SWMBO and myself.
2) Nice wines - wines I like that are doing something more interesting than the house wines or that are harder to replenish. Generally from $15-$50. Things like Corison's non-Kronos or the Jana Cathedral that have been on woot are good examples. These I am happy to share with friends who I think will appreciate them over the house varieties - I have a few close friends who are 'into' wine, though probably not as much as SWMBO and I are; I'm comfortable opening these for them. Sometimes bottles in this category get opened for non-wine-people if we're hosting a dinner party and we think they would go particularly well with whatever we're serving. Usually though, these end up being shared only between SWMBO and myself.
3) Expensive/Rare/Mailing List/older wines - Over $50, or hard to get. Basically, for SWMBO and I, these are Aubert, Sea Smoke,(some) Turley, Saxum, and Linne Calodo (which didn't used to be mailing-list exclusive, but I think it is now, though I'm not on their list and am just drinking down the remainder we have in the cellar). Also bottles like Corison Kronos, older CdPs and Bordeaux, Krug/Dom Perignon and the like. We rarely end up opening these for just the two of us - my parents and grandparents are both very into wine, and these are usually opened during family gatherings/holidays (we don't have a large family, so generally get-togethers/gatherings only include those people that would be appreciative of the wines) when everyone can share them and we can hear everybody's thoughts on them. Talking about what we taste in each wine is often just as enjoyable as drinking these ones.
So... yeah. That's probably a bit-wordy and category-heavy, but is a pretty good approximation of how we generally do things.