I keep my mac backed up daily with time machine. Is that a good enough back up in case something happens when I install mountain lion?
I for one wouldn't use Time Machine to backup before an OS update: if things were to seriously mess up, time machine would not be accessible and make that backup useless.
Use a 3rd party backup with something like SuperDuper (and make sure it's on an external drive).
But overall, that's just to be safe.
There are more Mac users that have the latest version than any other OS, that wouldn't be the case if upgrading the OS was such a hastle (as it is with certain other OS's )
I wouldn't run a major update without at least two backups. Never depend on a single backup, whether it's TM or a clone.