Hm. I've never seen that prompt. Do you have iCloud backup on?
And was that a cable to iTunes update, or did you do an over the air update? Because I remember iTunes giving me that prompt on my Mac, but not when I did an over the air update.
iClould back up is on on all our devices. I didn't think you could do major releases over the air (although could be wrong), so it was done with Cable connected to my Macbook air for my iphone and my wifes macbook Pro for her iPad.