Depends where I crossed it. If I was trying to make an opening in the line where some of the asswipe union officers were strutting -- weaklings who collect fat dues for years, rarely push legit grievances, mostly defend the slackingest slackers when they do and would sell out their mothers for a maybe-sorta promise about the future from management -- I'd probably cross. If I saw some co-workers who are good people and were in no better shape to go without a paycheck than me, I wouldn't have the heart.
Similarly, if I got off the elevator and the first people I saw where the scumbag executives who have gotten us to this point -- as a paper on strike, presumably, and an industry in crisis -- I'd probably turn around and walk out again.
In other words, I'm with Buck: Hard to answer until you're in that situation yourself.