Think of bedding stringers like adding a footer.
Plywood on edge hard against the hull causes a point load. Adding PB fillets along side it doesnt really grab the plywood nor protect the hull.
Elevating the stringer and put foam under eliminates the hard spot. Thats 1 way, but the PB fillet is only along and grabbing the sides of the stringer.
After numerous discussions/posts, over a period of years, many @ iboats forum adopted a different (IMO a better) method.
Elevate the stringer 1/4in off the hull, by whatever means, every so often. IE an inch long strip of 1/4in foam every foot.
Fill the gaps between the 1in long strips w PB and a PB fillet along both sides of the stringer. Once cured, pull the foam and infill those small gaps w PB & PB fillet.
Using that method, you now have a 'footer' of PB thats wider then a piece of plywood stringer on edge. A wider foot made of PB that fully encases the bottom edge of the plywood is significantly less likely to allow a point load/hard spot if the stringer flexes and the hull doesnt, or vice versa.... ya know like wearing snowshoes to traverse snow is better then a pair of snow boots......