If it seems certain that the friendly is going to die regardless, and my joining will get me killed as well...I'll pass.
If it looks like there is at least a small chance we can weazel out alive, I'll dive in and help.. then listen on ch1 about how I'm nothing but a cherry picker.. blah blah blah.*
Conversely, if I'm getting ganged by the masses and am certain to die, I DO NOT expect anyone to dive in, just to get killed with me. It's nice for people to dive in and scatter the conga line, but if I'm caught turning with a swarm it's my own darned fault. I'm always grateful for the help, and don't care about kill stealing...just please try to avoid killshooting.

*As an example: couple days ago, I dove on a friendly field where exactly 7 knights were vulching 2 or 3 rooks. I got 5 and the vulchees managed to get the remaining 2, as I corralled them on their escape attempt. Soon, ch1 buffer had the comment: "You're nothing without a 5k alt advantage, Steve." Nevermind that the guys were vulching(which means you really don't have any right to complain). I asked him what he would have done, 7 planes in the air against his one..... I could pratically hear the silence as his whines evaporated off the channel.