Sadly, speaking as a dinosaur, I have to say that once kata is altered to incorporate personal preferences or different teaching influences,then it is no longer the same kata vis-a-vis that which was originally taught.
Sorry for the slight thread drift but its an old hobbyhorse of mine although I am not saying it is an unnatural development. I recognise evolution happens, Chinese whispers whatever you like to call it or describe it and so in the same way does bunkai change and evolve because thats what happens as people pass things on through the generations, or change things because they want to develop their own style.
This has the unfortunate effect of people complaining that others can't possibly be doing a technique the right way because they themselves have been taught it the true way. Ho hum.
Anyway, to the point of your original question....Yes, I teach bunkai, Yes, I try to teach it as I was taught. No, I dont allow variation, nor do I allow students or instructors to vary.
I thought Simon's points very good although I am afraid I havent read McCarthy's book, and on that basis I would only agree with his points 2-4. For me, point 1 should always be to assess your opponent at that moment in time, to assess your own position, to assess the likelihood of attack and where it might come from ie punch or kick, from left or right limb, or even a disguised attack as has been suggested ie a grab not a punch. Once you have evaluated this then you can make the correct application to defend against the attack as you perceive it happening.
Clearly, the more advanced you are the more applications you have in your gunny bag so that you are not taken by surprise.
Osu
Trevor
Trevor Gilbert
("If I had to select one quality, one personal characteristic that I regard as being most highly correlated with success, whatever the field, I would pick the trait of persistence. Determination. The will to endure to the end, to get knocked down seventy times and get up off the floor saying "Here goes number seventy-one" - Richard M. DeVos)