I back up everything. I use Stablebit Drivepool with duplication for all of my source code, media, photos, documents, music, books, laptop backups, etc. I back that up periodically to a Drobo DAS and 8 Bay USB enclosure setup under Drivepool. I also have off site backup (working on a new NAS which will be accessed over a VPN). I don’t want to spend the time worrying about loosing anything I have put time and effort into. Been there and done that. Drives are relatively inexpensive but can fail without warning.