It is advised to backup everytime you changed/added anything important, like source code or new dokuments, like right after you did the changes (because after this change is = before your future change).
I personally never backed up single preferences or fonts or that stuff alike. Only folders, partitions, complete installs and so on. And i did it regulary. So, e.g.
in the 90s i had SCSI Zips and Syquests, that usually holds e.g. the complete (bootable) copy of one of my machines system partition. Or some folders with documents etc.. When i decided to backup my develop partition, most of the times a CDR was only 25% full, including all pictures and textdocuments i hoarded over time.
So, to fill the space up i used to burn all previous backups (from those external drives) onto every new backup CD as well.
I still have 99% of everything i had 25 years ago. Including complete bootpartitions, disk images and so on.
Today i have CFs and once in a while (see above) i clone the whole thing or just a partition and shovel it over to my fileserver and also external drives, zipped.
Most installs are present as clones in UAE anyway...
All my older CDRs are ofcourse imaged and saved as well.
The thing is: Make at least a full backup once in a while and store it separately, on a different external drive (or usb flash or whatever). and don't overwrite older backups, just make a new one everytime, but save space by not saving all stupid things over and over again, e.g. wallpapers you could easily get again from somewhere else (your cloud for instance. thats what it is for: wallpaper storage).