People usually recommend burley blends for breaking in since they are said to build cake faster than other types. But the same goes for virginias. I tend to break in pipes with whatever I think I will smoke in them eventually. Nothing like a dedicated pipe that has ever seen but that one blend. However, as you will probably want to experiment with different blends anyway, it doesn't make that big a difference in the end. Finding a right coupling between a briar and a weed is, in my opinion, far more important than any puritan musings.
As for the cakes, they will develop one way or another and I am quite convinced that past the first dozen bowls the actual thickness plays very little role. It is, rather, the chemical composition of the briar (i.e. the amount of sap and residue it contains) and the treatment it has received from the maker, that determine how long a particular bowl will take to reach its optimum smoking quality.