Oh, and since no one's yet weighed in on the actual question:
I started lubing the canvas prior to painting into it last Winter, as opposed to mixing the Maroger into the paint like David Leffel teaches. But I only do it because that way the paint pile stays open and usable for a couple days if I cover and refrigerate it. An interesting side note (like you needed another one): I was told that Odd Nerdrum, Rembrandt's red-headed stepchild, grinds his medium into the paint or mixes it into the piles. He says "that's the way it has always been done".
But then again ... he's probably opinionated, too.
Minh