I determine which posts to read by their read/unread status but I have a number of feeds which I don't need to read very often - I tend to let them accumulate and then browse through picking out the interesting stories. This means that my 'unread' total grows when, in fact, I have read everything that is currently interesting.

If I could set these feeds to import as already read, I would still know that they were there and go and check them periodically and my 'unread posts' would actually be the ones that I am interested in checking as they appear.