I would always stop if I saw an accident! The last time it happened, I saw a taxi change lines straight into a guy in his work van... I was directly behind them, so I stopped and offered myself as a witness. Good thing too, the taxi driver was starting to crap on and I just said "Don't try to say it wasn't your fault, I SAW you - you drove straight into him" and then he shut up. Never got called about it.
I probably wouldn't stop if it looked like there was other people helping, the last thing you need is heaps of cars all stopping and blocking the traffic.