I was looking at a welding forum that had a big discussion on this topic. All the "welders" on there were freaking out answering ones guys question on how to do this properly. Sounded like a bunch of union guys... taking about using gas detectors and , plumbing argon into the tank, getting saftey officer to sign off before welding.
I dont think you will ever get anyone to certify a proper proceedure for cutting into a propane tank, ... but in the real world we all know that its done all the time and there are 1000 times more succesful stories than accidents that happen.