I found this PDF document which addresses the issue, an unnumbered technical bulletin issued by the Cedar Shake and Shingle Bureau (the industry standards body) in March 2007. Apparently It's a matter of some controversy: people are installing tapersawns in three-ply applications (e.g. 24 inch tapersawn at 7.5" exposure) without felt interlay because it is aethetically pleasing, less expensive than using three-ply with felt, and they fear the felt slows down evaporation and possibly leads to trapped moisture. But the method is not recognized by the building code at the national level, and so the CSSB does not sanction it. Some jurisdictions are allowing it.
I cannot find the original at the CSSB website, so the link above is to a third-party website.
Applying a tapersawn shake in a 3-ply fashion and using a felt
interlay has raised concerns regarding the trapping of moisture and
the potential to shorten the life of the roof. The key discussion
point is that a tapersawn shake is sawn on both sides just as a
shingle, but a shingle does not use a felt interlayment system
regardless of edge (vertical) or flat grain content. Building
officials MAY approve eliminating felt interlayment between tapersawn
shake courses when tapersawn shakes are applied at weather exposures
of less than 1/3 the total length (3-ply roof). ALWAYS check with the
local building official for project approval PRIOR to deviating from
standard application guidelines. Check with the manufacturer for
product warranty requirements.
The 3-ply application of tapersawn shakes without felt interlay is
NOT an official method approved by the CSSB or building codes.