I've always stopped.

Two weeks ago (maybe last week, actually) I got a flat. Ended up walking the bike home. Had 5 cyclists pass me with nary a word. Did get some harsh glances. I think the harsh glances were because I was walking against traffic. If my butt had been on the bike I'd have gone with traffic. Oh well.

Karma didn't pay off then. That's not gonna stop me from stopping for others in the future, though.

I once donated a CO2 cartridge to some guy on the MUP. Nice guy.