Backups [Noob Question]

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http://forum.xda-developers.com – ive read in the other threads that we should be doing backup when flashing kernels. how important is it to backup for just flashing a kernel? is it even worth it? Im gonna be flashing Siyah Kernel I heard nandroid only backups phone settings? did i understand it correctly? Is it better to root first before doing backup? or should I unroot my phone first? Please tell me if I got it right Titanium = app backup? cwm = nandroid backup? a bit offtopic but does anyone experience battery drainage when connected to wi-fi even when not doing anything like surfing etc.? (HowTos)