quadcopters — should the propellers/motors all be at the same height
Tandem rotor helicopters have the aft rotor elevated to keep it clear of the front rotor's downwash and improve performance in forward flight (at the cost of reduced performance in rearward flight).

Most small quadcopters achieve substantial pitch angles in forward flight, often in excess of 30 degrees, which keeps the aft rotors out of the downwash anyway. Therefore, your proposal changes nothing in hover and fast forward flight, improves performance somewhat in slow forward flight and reduces performance in rearward flight.