I had a bit of a debate with a guitar buddy. I told him that after you change or touch a tube, you should wipe it clean with isopropyl alcohol. He fiddles with his tubes constantly, and he says he has never had a problem. Changing them in and out and buying "vintage" tubes that sound the same to me. The advice I was given in 1978 has stuck with me: "If you get oil from your skin on the tube, it can create a 'hot spot' that can affect performance or even destroy the tube" Is this true?