Knowing when to drop in on a con someone else is working over can be a bit hit or miss.
If I've got altitude and I see a con running out and no one closer than 800 yds and losing ground, I WILL dive in because I'm the only one capable of getting into guns range. Even if I can't get the kill because the target breaks, that may still allow them to close in and finish him off.
If I see a friendly being trailed, I WILL move in if I have position regardless of what the other guy says. Case in point the other night a friendly was making a shallow climb out of a furball, with an N1K coming up under him. The friendly was Check 6ed by another pilot, and he was waved off by the friendly trailing the N1K saying he knew it and was going to reverse. However, as I was already in a position to drop and circle in behind the con I did so. Before I was in guns range the target broke and the friendly he was pursuing came around over the top. I held back to give him the shot, but continued to follow the N1K. The reversing friendly was shooting from relatively high-deflection and missed. The friendly repeated he had the con, however I already had a guns solution (full plane, right on his wingroot) so took my shot and advised him I was doing so and took out the Nikki. As I said above, no negative words were exchanged, but even though the friendly "claimed" while still alting out I had the better position throughout, so took my shot when the friendly missed his. Probably the better strategy would have been for him to simply rope the other con as I could have more quickly shot him off.
If another friendly is working over a con but I've got the better angle, I'll take the shot. I try to let him know I am, but sometimes that slips through. Usually, if you've got the better shot at a target the other guy will let you take it if you let him know. Sometimes, though, you only have a narrow opening so have to take it when it comes up. I still try to let the guy know why I took the kill afterwards (usually an honest "Sorry for the steal, but he gave me too good of an angle" is sufficient. Most guys will understand).
What I try to avoid is conga line of guys all trying to jump out in front of each other to get the shot first, or spraying wildly to put more bullets in first.