Where to run power cable for hidden amp

As I mentioned in my first post, I don't want it to be connected to my stock A.M. radio. I want it to be separate. From what I see online, you need to run the power cable directly to the battery (with a fuse). Amps, including mine, have a remote input and I could put in an on/off switch to that which would tell the amp when to turn on. I was just wondering if you had a way to run the power cable straight to the battery would it in essence be providing juice the whole time or just when the key is turned on.