Putting grease over the chamber IS the most commonly given instruction. Hell, practically every loading video on line says to do it. I was told to do it back in the day. But, it is only after you shoot a while that you begin to hear about other methods. Just watch Cabella's video for an example. But, as my previous post suggests, there is a time for everything. I teach my students to grease the cylinders. I show them conical bullets, but offer them round ball bullets to actually fire. I don't even mention paper cartridges. In time they may try loading something different after doing an investigation on their own, but as a beginner they need to learn the safety basics. It has only been in the last 20 years or so that I first began to see wads being used to any great extent. That's when I started to use them.
darkgael is right about tight bullets being a seal, and I think that historically this kept the chambers safe enough.