You can see the state of a slide switch, but they require either a PCB mount, or a square hole in a panel, which is a pain to deal with. Pushbutton switches also have their uses if you have space constraints, but you can't easily determine is current state just by looking at it. In addition to the power switch, I will often put a small, low power LED on the robot somewhere that also acts as visual verification that the power switch is on. Try to put this on the computer board if possible. Use a relatively high valued resistor if you are concerned with power consumption.
Most toggle switches mount in a small round hole which can be made with drill motor and bit.
Your switch should be wired to your power source, so that when the switch is off, all power to the system is shut down. Many parts, especially motor drivers, have a nasty problem where feedback through various ground paths can cause them to engage. The last thing you want is for your motors to turn on at an unexpected time. Switching the power from the positive lead from your power source is a good way to ensure that this doesn't happen. New switches are, to be honest, ridiculously expensive.