I think Duc was trying to come up with a universal grading scale,
Yes, I see that. But then it must be quantitative.
I have a cue right here that had what most people would call a "taper roll". It was hitting close to 3mm But not quite) variation at the center of the shaft. I spend a little time this evening bending it back, literally, and now it is straight. Amazing, isn't it.
Now, it's a "cheap" cue, and not one that I will try to sell, it's just something to play around with. But you can actually do this with a lot of cues, I have done it before.
I have one that is "S" shaped...that I'm not sure about.....

Without that, anything other than straight is just bent.
I really am not so picky about it, but some are. It's the main thing that makes my hesitate at all about selling here. I have one for sale here now that is straight. I mean it's straight. But the way some people are I feel like I have to get up at 3AM to check it just to make sure it has not changed. I was so paranoid about such things I made what might be the longest for sale post for a single cue in history. I talked about everything and measured everything.
Anyway, IMHO any valid method would require measurement.
In my signature it says "Bury me with my JOSS". Should it say "Bury me with my bent JOSS" just in case somebody calls me on it? Both shafts have been exactly the same for over 25 years, bent, and I ain't worried about it. If I ever decided to sell it, which I won't, I would expect the appropriate money for such a cue. Meaning it is simply among the best playing cues ever made, bar none. My gawd, if you unbent it, it might lose the mojo! LOL!