Bob,
Theoretically, it shouldn't matter, but in practice, some balance between LE and TE usually "feels" better.
If the wing has a spar a bit forward of half the chord (LE to TE) length, you could line up on that. If not, you can stack them with about the same sweep as on the finished wing, more or less.
Doing this keeps the automatic taper (cut in by sanding thick root rib to thinner tip rib) a bit more uniform. Alternate the ribs as suggested, and sand the ribs as suggested, to put their edges smooth to the covering instead of a single sharp edge from the stack-sanded taper.