I find that either a server needs more space, or has files that can be deleted. For the former you just increase the disk space, since most things are VMs these days and increasing space is easy. For the latter you can usually delete enough files to get the service back up before you start the proper cleanup.

If you really need some reserve space (physical server), I'd much rather store it in a vg (or zfs/btrfs subvolume). Will you remember the file exists at 2am? What about the other admins on your team?