If you are referring to computer accessories, you definitely want them on a regulated 12vdc, not straight from the car's electrical system (which fluctuates wildly). If these are non-pc accessories, the fuse box is a good place to get constant and switched 12v from. HOWEVER... When connecting a wire, make sure you put it on the hot side of the fuse holder. In other words, take the fuse out and see which contact has the juice. Connect to that one. Otherwise, whatever you connect is going to add to the circuit that fuse is protecting and likely blow that fuse. Example: you need a constant power source, you find your headlights fuse always has power and has a 20amp fuse. If you connect on the fused side, when your component is on, it's current (say, 5amps) + the headlights' current (say 15amps) is what will go through the fuse. You may blow the fuse and lose your headlights. But connect the component to the side wired to the battery and it will have constant power but NOT add to the draw in the fuse. Since you said you would have inline fuses, you'll still be protected no matter what.
Also, consider using the 12vdc provided by your power supply as your regulated 12v for other things. A good example is many people on here connect the power wire of their lcd monitors to the computer power supply. This way the monitor is turned off with the pc, plus it gets a regulated 12vdc. Food for thought.