One more tip to factor into all that has already been said:
When you initiate a flat scissors defense against someone, there will be a few points where you are in that guy's gunsight for just an instant.
Therefore, if your attacker has strong guns, capable of killing with a brief snapshot (i.e., has 2 or 4 cannons or has a spud gun and knows how to aim it properly), you are putting yourself at a greater risk than if all he had was 4 or 6 machine guns.
When suitable, I rarely try a flat scissors defense against Nikis, but I'd almost certainly try it against a P-51B or an FM2.
-Llama