I don't have to wipe cache and dalvik, when restoring a backup do I?
Only thing I've ever had to wipe when restoring something on my phone is the widgets on screen. If you backup the layout of your various screens and when you restore it the widgets, or some of them, are in the same place, it might double over them, which will drain battery faster and possibly slow down the phone a bit (doubtful on the Bionic though). Not sure what dalvik is but I know everytime I've restored something pretty much all I did was hit restore w/ TitaniumBackup.
If you are talking about a nandroid backup/restore, I do wipe cache and dalvik in recovery before starting the restore. I don't think the wipes are required, but my restores have always put me back exactly as I was before the backup. If you are in recovery anyway, the cache wipe takes about 1-2 minutes and dalvik is virtually instantaneous. Better safe than sorry.
Try it and let us know. What I was saying is if you backup your widgets, make sure there's nothing already in place when you restore, otherwise the backup might just overlap the existing widgets and you'll have two, which will drain more battery and potentially slow down the phone.
I'm not sure what a Nandroid backup is TBH, I just assumed you meant Titanium or AppMonster or something, in which case I've never done anything but hit backup/restore.