I owuld recommend you run an 8 guage power wire off the battery (fused very close to the battery back to a mini fuse panel. You haven;t stated what all you want to power but if it's carputer power supplies, amp etc, they all need constant battery power as well that is switched by a lower current line from accessory.
Picking off an accessory line is very vehicle specific but. Maybe if you post your vehicle specifics, someone can help with that. Quite often it can be found at the steering column plug in the drivers seat foot well.
If you do decide that all of your power requirements are from acc powered then I would still run the same power wire from the battery and add a relay to trigger power through it from the accessory circuit.
Nasty things can happen when you over tax the factory wiring.