We have been cutting down on the plates we keep. Before I started they kept everything, even dated flyers that could never be reused as they had a date on them.
As for files, I like to keep everything. Hard drive space is not a big deal and they are cheap. My files go back years and years. Every so often we get someone who printed something 10 years ago and wants to do it again or use something from that file, nice to be able to pull it up and be able to do something with it.