Cedar and redwood have mostly water soluble extractives. Oils, tannins, phenolics, etc. When these surfactants come out of the wood (moisture driven) and come into contact with the spun-poly housewrap, it can ruin the pore structure of the housewrap.
For cedar and redwood...the big problem is that most building envelopes push moisure through the exterior walls...from the interior living spaces outward, to the exterior. In older houses and in today's standard tract house, it's more prevalent. On better-built houses where the envelope is carefully detailed from basement to ridgeline, moisture mitigation is less common. It still occurs, but at lesser rates.
This moisture will pass through a spun-poly housewrap. It next encounters the back side of the cedar claps. It may condense on the back side (not the show side) of the cedar siding. If the back face of the siding is untreated, unsealed, unprimed, the cedar will absorb the moisture. As it is absorbed into the wood (and even if it just stays on the wood surface) it will dissolve the water soluble extractives in the cedar. If the moisture does not condens out and remains as a vapor, some will pass into the clap, some will pass between the clap overlaps into free atmosphere.
Back to the dissolved extractants...
These extractants, some of which can act as surfactants, can adversely affect the housewrap. The surfactants and polyphenolics can create a bit of a problem with the micropores in the poly housewrap. The housewrap eventually loses its ability to do one of the jobs that it was designed for...passing water vapor. If the housewrap can't pass water vapor though, the housewrap instead ends up acting as a condensing surface...and may hold the condensed water between the housewrap and the sheathing, against the house sheathing. Not good.
Some moisture that enters the back side of the siding passes all the way through the siding, carrying the extractives with it. When the water hits the show side of the siding, it will evaporate. The extractives are left behind on the surface of the siding. This can result in anything from subtle blotchiness to outright, very obvious brown or reddish-brown stains.
Ever see a cedar clap house which turned blotchy within a year or two of being stained? More noticeable with transparent or semi-transparent stains...that is a good sign that the siding was never backprimed. It also depends on how the house envelope handles moisture.
So...if you're installing nekked cedar, either use tarp paper instead of a spun-poly housewrap, or install the spun poly with a rain screen. Simpler to go with tar paper.
If you're going to coat the show face of the siding, backprime it before you install...no matter what housewrap you use...tar paper or spun poly. Moisture can pass through the backside of claps that have not been backprimed, and as it passes through to the front it can deposit the extractants, causing blotchy stains on stained cedar. If the cedar has been painted with a film-forming finish, the moisture can literally lift the paint right off the front of the claps and cause pretty amazing water blisters.
In the end, just keep nekked cedar claps off of spun poly products. Many builders are opting away from spun poly and going back to good old tar paper (or Roofer's Select...even on the walls) as a result.
It's not as much of a factor with cedar shakes or cedar shingles, as often times these are installed over furring strips or a product like Cedar Breather. this is done to promote better drying of wet shingles and shakes, which helps to avoid cupping and subsequent cracking.
Sorry for the length of response...and I hope this doesn't just confuse readers even more!