Biology and physics dictate that this be true, regardless of purely tactical advantages.

When maneuvering the pilot is subjected to what is called g-forces, which cause a perception of change of mass. You can feel it when you’re in a car and hit the brakes: your body wants to move forward regardless of the car stopping and you also get the feeling that your body is heavier than it normally is. A more interesting aspect of this phenomenon is when you’re driving at a high speed and reach a steep downward slope. You get the feeling that you guts want to push upward into your neck.

For the pilot of a fighter at top speed (upwards of 550 kph for later-model A6Ms in level flight^[1] ) the same phenomenon is increased many times over. Your body can feel 2 times, 3 times, 4 times heavier than it really is (in contemporary fighters the max g-force can routinely reach 9). The crucial problem, however, is not the pilot’s body itself, but the blood within it. As it can move freely, it will tend to press wherever the g-force dictates that it does.

This is a loop.

When the pilot starts moving the plane up, the g-force will act towards his legs (opposite to that car going down the hill) and that’s where the blood will want to pool. But this is something one can counteract by appropriately working the muscles in one’s abdomen, buttocks and legs. You cannot completely nullify the g-force, but you can make sure the brain receives enough blood to keep working. If it doesn’t, you experience a black-out, you lose consciousness and you likely crash and die.

The opposite situation—when the blood rushes to your head, pushed by upward-acting, or negative, g-forces—is known as a redout. It is, in fact, much more dangerous, as it can result in eye damage (that’s where it gets the name from: the blood pooling in your eyes makes you see red) and brain hemorrhage. That’s why the pilots would avoid negative g whenever they can.

Edit: It occurred to me that I ought to clarify why negative g is so nasty, besides the damage it can cause. While there’s many places in the body below the heart where one can squeeze to stop the blood flow, there’s actually very little above the heart. The neck? Well, you wouldn’t want to squeeze there. As a result, negative g is both more dangerous and essentially impossible to counteract. End of edit

Now try to flip the image above upside down in your imagination (or just click here). This is what the Japanese pilots (and other pilots as well) did. Instead of going ‘downhill’ and pushing the blood into their heads, they would invert their planes and, in a way, trick their own blood into acting as if they were going uphill, thus preventing red-out.

If you wanted to peel away to escape from the field of fire of the defensive machine guns, diving would indeed be a good option, purely for the speed it gave you. And if you wanted to dive, you would invert your plane first. And if you wanted to invert, you could indeed do it before firing so that you could come as close to the target as possible, shoot, then immediately dive, though to the best of my knowledge shooting while inverted in level flight was not a common tactic.

And yes, it was usually a good idea to attack a B-17E or B-17F from the front, where her defensive armament was weakest. This is why the B-17G received a chin turret.

However, it should be noted that there’s also a tactical advantage in general to inverting your plane before attacking, if you attack from above. As the A6M has a low, forward mounted wing (this was generally the case back then, it’s not just the Zero that was like that), the wing obstructs the pilot’s view downward. See here. It’s easy to imagine a ‘shadow’ being cast by the wing, covering a huge volume of air and area on the ground. To give you an idea of the scales we’re talking about, AVM Johnnie Johnson states that in a Supermarine Spitfire at an altitude of 5 miles the wings would cover ca. 1,000 square miles of ground^[2]. Therefore, if the pilot was about to attack a target below, he would scan the air above one last time, make sure there are no enemies there, and then flip the aircraft upside down to give himself a chance of doing the same for the space below.

Notes

[1] Krzysztof Zalewski, ‘Japońskie lotnictwo pokładowe’ Lampart no year, p. 137.

[2] AVM ‘Johnnie’ Johnson, ‘Wing Leader’, Goodall 2010, p. 52.

edited for typos