When to backup before jailbreaking?
Jailbreaking at least the older generations of iPod Touches shouldn't erase the contents.

However, there's always the risk that something goes wrong in the process and you might have to restore the device - either from a previous backup or to set up as a new device with no existing data. There's no explicit requirement to take a backup before jailbreaking, the jailbreaking doesn't need one per se.

Generally, I'd say it's up to you how fresh you want your data to be. If you are willing to lose the data the device has accumulated after the latest backup, it doesn't matter how old the backup is.

I recommend taking a backup right before jailbreaking.