What do you backup on TWRP nandroid backup

TWRP does a good job at labeling it under your device name (dev name/date+time/data.) but i did notice TWRP's clock sometimes gets a lil off. I set it to my GMT offset once and a week or so later i did a backup and i was upward 2 hours off. Not a big deal but thought it may be worth mentioning. Also, their battery% is off a little. Again, none of this really matters but just an observation.

And I keep several NAND's because its easy as pie to reload those and never miss a beat. Once i get a rom the way I want it I make a NAND and file it. I try to do that ever now and again so i have a recent update. Whats it hurt? You can always have a NAND of your favorite rom so you can switch pretty seamlessly...and you always keep an updated backup.

Space isnt hard to come by either...several 1GB files on a PC is nothing and even 2GB on a phones SD card isnt much to ask for given the peace of mind it offers you.