When it comes to pruning revisions, it would be good to keep the first x revisions and the last x revisions and remove the ones (if any) that are in between. I can imagine that the first edition of a post in particular, is possibly the one you most likely want to see again in some situations.
Please don't just prune the oldest revisions, but allow the admin to specify the number of posts to keep from both the beginning AND the end of the revisions list.