I too have this need for my site and I don’t think your above answer solves the problem described.

The Related Posts functionality that sits below an article determines what to render in that box by looking at the category and tags of the post being displayed. If the post is assigned “Category A” then related posts will be “Category A.” If the article is tagged “Category G” then related posts will be “Category G” posts.

Is there a way to set up something similar for a sidebar? With this setup, one sidebar could be created that would display related posts that match the category and/or tag of the post in view. Otherwise, if I have 40 categories, I’d have to create 40 different sidebars and make sure to manually select the correct sidebar when posting content.

Thanks for your thoughts on this.