Mine is on all the time and I have never been hijacked. The only time I turn it off is when I'm out of town. If your computer is off there is no need to turn your modem off. They can't hijack a computer with no power to it. Also, if you have an external dialup modem and you are disconnected you don't need to turn it off. Nobody can call into your modem.
Also, if you have dialup and you aren't connected nobody can get onto your computer either. See the comment on hijacking with no power.
