Read every post, no. But at least get a general idea of the posts for the sites I subscribe to. There are a couple of sites that I let over lapse, HN is a good example, but for the most part, I atleast read the headlines / tags of posts for sites.

One nice feature of newsblur, which I don't use often and not sure if it is available in other readers, but the intelligence trainer would probably help in instances where you have a lot of sites and don't necessarily bother reviewing all posts.

Maybe I just use RSS readers differently. But if I were just going to periodically review what a site has, I would probably go to the site periodically rather than clutter up my list of feeds.