The short answer for Part 91 is that if the item is not a required item under some regulation or other then it's OK to depart as long as the defective equipment is placarded as inoperative and switches or breakers placed in the appropriate position. Inoperative could mean the switch is in the 'O F F' position.
Part 135 is different. Under 135 without an MEL that covers the item then everything in the aircraft must be serviceable to depart.