A cleaning algorithm would be useful...but...I wouldn't trust it :-)
a year or so ago I undertook a project to clean up my music, re-rip it into iTunes, scan cover art and have appropriate meta-data. I was almost 90% when my hard drive died. Haven't gone back to it yet :-)
I think it would be worthwhile for me to go back through my delicious bookmarks and retag and add abstracts. I "only" have about 2000 bookmarks. I think that what I will find is that about 500 of those links are dead, which makes it a good excuse to clean things up ;-)
As far as going back to old blog posts. I have considered it, but with about 3000 posts (across 3 blogs), that could be an issue. For blogs, I think it doesn't matter as much (the retagging) since one could do a forensic analysis on our blogs and could tell things about the evolution of our thinking. I write blog posts to outwardly communicate with others. Bookmarks on the other hand, I keep because I find them useful for my own personal use, the outward sharing is just a nice side-effect :-)