I was also taught to wait 30 seconds, and I passed that advice on to every person that I helped start into shooting. But I have never had a round cook off or experienced any time of firing due to a delayed primer burn, or anything else. But I still wait, just in case. I did once have a squib, which was an odd experience. But the sound and feel of that was strange enough that I was not foolish enough to fire another round until after I had cleared out the barrel.