When my character was younger and I did a lot of running (without the aid of invisibility), I would help the shopkeeper/cleric. I found that I usually did not land the killing blow anyway (if I could, I probably would not have had to run). I think it would be fairly easy to notice if someone was exploiting this since it would have to happen repeatedly to be of any real value.
Its not really nice to lead monsters into town, but on the flip-side, I guess even the most benevolent characters would run rather than face certain death at the hands of an inconsequential monster.
Don't put too much stock in what I'm saying though, I'm not a DM.
