There are no reported cases of the code being refused at check in as far as I know. Unless you have an absolutely stripped insurance coverage, you are covered by your car insurance. You have absolutely no greater risk with this rental code vs. your daily driving.
I believe that those who think you are rolling the dice are either folks who must follow the rules every time, and insist everyone else must or folks who cancelled their ressies when the debate about this code's validity was roaring. If you were to visit flyertalk, National has a strong reputation of honouring their codes.
I can tell you with absolute certainty that my car insurance does not cover any rental car at all. My car insurance is for my car, not for me. It's something I asked my insurance company directly before I rented a car at all (I am 27, so I've only been renting cars for a couple years). In fact, no insurer in MA insures the person, they all insure the car, and insurance for the car is required in order to register. We're also required to purchase "uninsured driver" insurance, so that if an out of stater hits our car and does not have insurance, the car is protected. Also, anyone can drive our car and it is still insured. This includes unlicensed drivers, so if someone steals my car and crashes it, the car is still insured. I have car insurance, but it only is useful on the car that I own. I can purchase rental car insurance if I wish as an added cost.
Because of this, I use my credit card's built in rental car insurance (which is a risk in and of itself, because my credit card doesn't cover nearly as much as my actual car insurance does).
My credit card's built in rental car insurance requires me to deny all insurance from the rental agency. If I do not explicitly deny it, then they are not responsible for any damage to the car.
Because the 67 code includes the insurance, there is nowhere I can initial to deny the insurance.
Therefore I am not covered at all if I use this code.