You are blowing MORE OXYGEN into the tank when You cut with a torch. Any unburnt fuel from the engine exhaust, and You are right back where You started.
Having said that, it probably won't blow. How luckey do You feel?
I met a guy who scrapped condemed propane cylinders for a living. He just torched holes in them, no water, no purge. He said on ocasion one would flare at the cut. He did this for several years without incident. Personally, I think He was nuts.