I prefer a used PC as well; I just find them to have a charm missing from an unused item.
As for dating, the most it does is set a "no later than" date for the printing that the mailed item came from. I don't think you can hope to quantify what % of them were postally used when issued, etc. Also, PC makers could possibly reprint PCs for years; they weren't issuing one-year sets like a gum card maker. Some of the restaurant promo PCs were made over long periods of time if the establishment had a long history, like Jack Dempsey's restaurant. Figuring out what is scarce and what is not is an art in itself. I've been collecting PCs for a few years now and I'm just scratching the surface. Even the established PC collectors are always finding new stuff.