I did mine straight to the battery terminals, I didn't want it through the ignition switch , suits me fine that way 
This is definitely your best bet - it's already switched to ignition and fused appropriately. Wiring it direct to battery would almost ensure a dead battery - somewhere down the line when you forget to switch the unit itself off.I piggy backed mine off the USB socket feed under the seat. That way it's both switched and fused separately.