It's not just about Windows 8.1 - you should never make any updates or major changes to your system without have secure backups of any important files (either system files of user files). You should maintain up to date images of your entire system drive using an imaging application such as Macrium Reflect (free! so there's no excuse!) and individual data files should be backed up to external storage (500gb external usb hard drive for about £30 or less). It's not only system upgrades that put your system at risk - any changes, installs, updates, malware infections and especially a hard drive failure can lead to loss of everything at a stroke. System imaging would also enable full recovery in a matter of minutes if your upgrade to 8.1 did cause a problem.