that's all too true. it goes both ways, but in tango the deck tends to be stacked against the ladies (traditionally) and despite the "code" being a bit more flexible here in the US (or outside Argentina) I tend to err on the side of caution when it comes to feeling comfortable approaching someone who is not already a "regular" on my dance card.
Me too... unless they seem to be new to the event and clearly don't know people. Then I might approach them for a dance. (I got to dance an unforgettable tanda awhile back with someone that I thought might be a new beginner but who turned out to be a visitor who was the organizer in his community and a teacher at regional festivals!)