If you vent it out to the point where there is no more LIQUID left, then purge all the remaining GAS and AIR out with water, then you'll bee ok to cut. You can drain as much water need to do a small cut just to settle your nerves. Bottom line is there has to be vapor and some oxygen in there to combust so if you displace it with water, then you are good to go. The propane doesn't seep into the steel and come out later. It will become a vapor at atmospheric pressure so venting the tank for a while will work. You could go so far as to dump a shatload of baking soda in it and then pour in some vinegar. This will produce carbon dioxide and water. When the bubbles reach the top, you are purged of any O2 and can begin cutting..
The bubbles may not make it to the top but the CO2 will... Put a mouse in a shoebox just above the vent hole, when he kicks the bucket, you have CO2 all the way to the top..