So when you first time open your iPhone it says you to connect to iTunes. Does it install newest firmware on your iPhone when you connect it? I have 3gs and i havent connected it because got it yesterday and i would like to jailbreak it.
It will not make you update. Come on, you run the computer the computer does not run you
In order to upgrade you need to click "Download and Install" then sit there for 7-10 minutes while download completes then 7-10 more minutes while iTunes installs it.
If you make a mistake, just unplug the phone while the IPSW is downloading. After the download is complete, iTunes will just stop and wait for "you", not it, to tell it what to do next.
My point is, unless you are in a drunken stupor, you cannot accidentally update.