Before the voltage regulator. This way, the fuse will open if you have a short in the voltage regulator itself, as well as downstream of the regulator.
Elsewhere in the circuit depends on the circuit. You can have more than one fuse. If you expect that some part of the circuit is likely to short, you can fuse it separately.
Motors like slow fuses, because motors can draw large instantaneous currents as a part of normal operation.
P.S. Fuses typically protect from fire hazard at large, as opposed to protecting the appliance from damage. You need to consider specific failure modes that you want to protect against. For example, if you're considering short circuits, you need to consider various specific places where they may occur (internal short in a motor, 12V shorted to GND, 5V shorted to GND, B shorted to A, etc).