Backup questions.

Hey guys im currently testing some customs roms, since stock roms after A8.1 suck and i would like to know how to backup my phone.
I am using TWRP 3.5.0_9-1 Unofficial and in Backup menu i can see many partitions.I thought i should backup Boot/System/Data but im not sure.
I would also like to know if i can backup a stock rom the same way.