Is it a good idea to stack naval bombers in stacks of more than 10?

There is rarely a good reason to have more than 10 of the same type of plane in an air stack. Two stacks of 20 would be ideal.

Since only 10 of the planes would attack, the only reason for stacking more than 10 is to spread the return fire damage around. Only 10 attack but all 20 take damage.

20 points of AA fire is divided among the stack....20 vs a 10 stack or 20 vs a 20 stack

It's generally not useful. If you were hoping to conserve HP and keep your naval bombers alive, it makes no difference using a single stack of 10 or a single stack of 20; both do the same damage to the ship so this is a constant. In terms of the damage the bombers receive, consider this:

-Let's say say you have 20 naval bombers than together have 400 HP (20 HP per bomber). Let's also say that the enemy fleet deals 10 damage for every time you attack, and that you need 3 attacks to sink the fleet.

Let's compare the results between a stack of 10 NBs and a stack of 20 NBs, which both will deal the same amount of damage:

1) A stack of 10 Naval Bombers (200 HP) sinks the fleet in 3 attacks and loses 30 HP in the attack, leaving it with 170 HP. It lands with the other 10 NBs and the damage is then distributed equally, leaving the NBs overall with 370 HP.

2) A stack of 20 Naval Bombers (400 HP) sinks the fleet in 3 attacks and loses 30 HP, leaving it with 370 HP.

For faster damage you should split your naval bombers into two stacks with 10 NBs each. Just be aware that this means the fleet gets to defend twice as often.