I think the most important Bunkai rule is to know as many applications as possible and be able to do them.
If you train a kata and you don't know what you are doing, then you're wasting your time. The kata must become reality.
You must know that every little step/attack/block can be done in many different ways, in different directions.
One of the best karate teachers I met said once to me "You know a kata when you know at least 1000 applications to every step in the kata". Perhaps "1000" is too much but I understand his point. In bunkai one movement can be done in many ways.