I quit keeping a score when shooting 3D a longtime ago. Sometimes I do still carry a scorecard, but instead of keeping track of 10's and 5's, I simply write on the card K, W, or M.
I dont necessarily base it on the score rings either, as many of them arent anatomically correct.
At the end of the round, I simply want to see how many shots were clean kill shots. I often skip targets that do not present a shot scenario I would actually take, based on distance, positon etc. I shoot 3D for hunting practice only. I usually fair pretty well. Its more fun, and more realistic practice.