Many, many years ago when a big hard drive was really big but only had 20 MB space, it was more or less mandatory never to fill more than 80% of the drive.
As drives got smaller but with a lot more space, the rule did seem to hold. About 6 years ago it was mentioned on the website of Seagate. (I do remember this because I bought two 250 GB Seagate drives 6 years ago).
But nowadays I wonder how much space you leave free and why. With >= 500 GB drives more or less standard in every computer sold the last 2 years, leaving 20% free is about 95 GB. And on a brand new 2 TB drive that is 380 GB (and that is more than the total hard drive space in my laptop).
In my own server I have a software raid 5 system with four 1.5 TB drives and lvm 2 on top of it. I start to make logical volumes bigger if there is less than 5% free, so filling up 95%. <added> Or ... I start to clean up, remove old files no longer needed </added>
Please respond with your numbers, how much do you leave free and why? I really want to know if there is a "golden" rule for this.