Actually, I'm pretty sure that in Destiny there is no real "kill stealing" if I'm not mistaken. At least in regards to PvE. If you are nearby when something dies you will also get xp/glimmer for the kill. If you shoot and hit something, you'll get credit for a kill regardless of who dealt the final blow. At least that is what I have noticed myself so far. Only a few specific bounties do not follow this however, but patrol missions that require you do kill things or get drops of certain enemy types seem to follow these "rules" that I mentioned above.

Yeah, my specific issue was with "Kill x amount of enemies" patrol missions. I'm pretty sure the percentage does not go up with assists for people outside your fireteam even based on proximity. I could be completely wrong, though. Damn kids today and their Rock 'n' Roll music...

And for those Kill X patrol missions, so long as you get at least a shot in on them it counts. Sometimes for me if I'm even near the enemy when it dies it still counts. So that makes it easier and what the OP does actually helpful in many ways.