The DS-5 power supply is just a simple circuit that generates DC power for the Series electronics. There's nothing in there that will age or wear and, when your bass is not plugged in, there's no power draw at all. So, I think you can pretty turn it on and leave it on without worrying about it.
The original red-knob F2-B preamps (and I think the current black-knob ones as well) have the power switch in the same location in the back. If you don't turn it off when you're not using it, you are putting time on your 12AX7 tubes and using power too, so it's more critical to switch power there.
David Fung