I have been doing some research. It looks like it will automatically download if connected to charger and WiFi is on, whether you want it to or not. But it does look like it will ask permission to install. If you choose Later, it will annoy you every chance it gets!

Funny thing is I've never seen any of my iOS devices automatically download an iOS update. They may have and I just didn't notice, but I've always had to go into Settings --> General --> Software Update to force an installation. But then I generally update as soon as they are available.