I think the idea of pushing the contrast in the printing stage is due to the finer grain that comes from "underdeveloping" the negative -- something that is more important in 35mm than larger formats (assuming fine grain is the goal). I've seen lots of recommendations for "thin" 35mm negatives on the web, but none in books. It's probably one of those informal rules of thumb that used to pass between photographers verbally and now does so on the web.