I'm thinking parallel to the rim and perpendicular to the shaft of the tamper. I think the ember might be burning perpendicular to the ground, with gravity drawing the ember downward, but you still want the unburnt tobacco to be even across the chamber.
I've always suspected that the high side of a canted bowl catches a little more heat, but I think this discussion may be a little more exacting and precise (or trying to be) than the process itself. The ember will adjust itself as it burns, and I would just accommodate its way of burning.
A more vertical chamber is probably the most efficient, which is why most pipes get built that way, but many clay pipes are canted and they have been popular for centuries.
The pipe in the photo is canted so slightly, I think it can be treated like a vertical chamber.