The reason for covering the entire panel is simple- I've found out that these old cars and trucks might appear to only have a couple small dents here and there, but when you get it stripped down to bare metal and start running your hand on it, you find that there are little dents all over- especially trucks. So, that's why it is quicker and better to cover the entire panel, then sand it down until you start seeing some metal again. I've found that a good way to feel for dents is by putting on a brown jersey glove and running the hand flat on the panel. If there truly are just a couple dents, then I would not cover the entire panel.
