At the time I was doing mine, the gang on the Nat. site advised using a 1/4" cushion to prevent a "hard spot" where the wood contacts the hull. Since then I've heard of many folks who don't follow that theory, and haven't had a problem. I did, and had no problem either but quite possibly wasted my time doing so?
So I have been doing some reading on this - another one of those subjects with varied and adamant opinions!
Here is what I took away:
It isn't a "cushion" unless you use foam or the like.
If you PB the stringers to the hull, the stringer has full structural contact with the hull the entire length, thus avoiding a hard spot.
A hard spot is where the stringer puts more pressure on a portion or "spot" of the hull than other places along the stringer.
Plywood stringers stand a better chance of having a hard spot than timber as they are harder with less flex or give.
Now the important one:
The reason you want a gap filled with PB along the entire length of a stringer rather than having the wood touch the fiberglass in spots is that the PB will shrink over time, pulling the stringer that is already in contact with the hull hard into the hull creating a hard spot.
That said, it seems unlikely to cause a problem on my (our) 18' boats with timber stringers even if there is wood to glass contact, but I am going to careful about having a reasonably even layer of PB to bed my stringers.
So thank you for you post that piqued my curiosity!
Scott
P.S. at least the transom will be installed, and glassed this weekend. May even get the stringers bedded, but not glassed.