another super important factor is how travelled a given stretch of road is, and or what sort of shoulders are there?
yes yes...
This past Summer we relied on gMaps to route us, hiking in Europe. As good as Google is, it sometimes doesn't work. We ended up quite a few times on abandoned forest roads that have become impassable. Whenever I could, I would scan the itinerary with satellite imagery. Worked quite well.
WRT the Sierra Cascades -- This is all very preliminary and tentative. One great thing is that being in the USA, the amount of information is impressive. G Streetview has lots segments covered.