If you plug it in, the WiFi will stay on when the screen goes off, so you don't have to keep monitoring it to see that it keeps from going to sleep. Personally, I left mine plugged in for most of the update process, but that's not really necessary. I'd make sure you're fully charged if you're not going to plug in. My update took more than an hour, and while the screen does not have to be on that whole time, the processor will be doing some heavy work, so I'm sure there will be noticable impact on the battery charge.