It's completely optional. What I'm more concerned about than bandwidth (I have unlimited) is battery usage. They also give you the option to have it postpone uploading until you have plugged the phone into a charger. I do it that way. I think you can also have it only do it when you are connected to Wifi, but I don't remember.
It's pretty funny how they are ranting and raving about their feature in iOS 5 that allows you to use the volume button as a shutter button. This has been the case on iPhone for some time with 3rd party apps, although Apple kicked them out of the store because it would confuse customers.