I've tried a few different workflows, but I think I've come up with one that works for me.
At one point I tried using the unicode "★" character to filter ratings by priority. I also tried using various folders marked "read," "unread," "priority" etc. Neither of these worked very well for me. Ratings weren't very useful unless combined with status, but status folders didn't work very well for me because Paperpile doesn't display folders as clearly as tags within the citation list; however, tags are a problem because they are sorted by frequency of use and so a tag for important documents will appear low on the list even though it is actually the most important tag.
My solution was to use tags which start with a "#" sign. This means I can quickly filter out only those tags related to read stats or priority by hitting the pound key. Then it is easy to filter or change status. Currently I have: #priority, #toread, #someday, #done, #skip, and #inbox for items not yet sorted.