IMO, one should get the kill as an assist if you do more then 90% damage. I've seen far to often when I do 96% damage for example someone else does the 4% and they get the kill while I'm getting rez'd.
Even if the shoe is on the other foot and I'm the one who finishes the opponent and rez the team mate that team mate should be credited for the kill.
From what I've seen those who do 10% or less damage get the whole kill. That is not right and I've been on the receiving end of that 10%.