Working on a project recently I needed to do some stringers. Originally I tried the classic sweep 1 rail method which in some cases worked adequately but in some others it produced some not so good looking and rather bizzare twisting stringers. So I tried something else which worked really good:
1. I intersected my object with radially spaced planes as you would normally do. I didn't offset my surfaces though to account for skin thickness. Offseting a polysurface in rhino is a pain, it won't produce good results and in my opinion should be avoided.
2. Instead I took the above intersections and extruded them normal to surface with a distance of skin thickness+length of stringer that goes into the former. So if your skin thickness is 1.5mm and you want your stringer to go 3mm into each former that would be 4.5mm.
3. I offset the resulting surfaces into solids with the desired thickness.
4. Usually your intersections from step1 run along more than one surfaces. If that's the case extrude each one normal to its respective surface, then merge the resulting surfaces into one so you can offset it at step3.
So what you're having now is a bunch of (oversized) stringers which you can use to cut notches into your formers. Now the last step would be to trim those stringers back to the desired size.
That's it. I hope I'm not telling you something you already know here and someone finds this one useful. Oh, and it's always nice to see a rhino thread back at the top.