I will not disagree with the various techniques involving poles and strings and such. They work fine, as long as you recognize the problems, such as the sheers not being shaped correctly, natural curl in the string, etc. Honestly, I did the pole trick.

I also stood back from the boat's wired together hull and looked at it, long and hard. If one of my sheers is high on the plywood, it could make the poles equal, but still have a twisted hull. So I used my eyes as well. I also examined it for bend and curl. I don't want it hump backed, and I don't want it bent and inherently turning. Btw, I missed a minor hogging that occured near the stern.

Does it affect you and do the boats have these problems inherently? Yes and yes. But don't read into that what isn't there. Any time you do any manufacturing, there are tolerences. The most perfect looking part is imperfect if you examine it in enough detail. Every kayak made has twist, as well curls and bends in it. Some more then others of course. I doubt there is any hard and fast rule to how much they affect you. For much depends on the environment the boat is used in, and who is using it. For my kind of paddling (duck boat type stuff) I could probably live with lots of twist without a problem. Heck, I could probably have the stern 90% off from the bow and still get along fine for duck boating.

I would suspect that twist affects hard chined boats more then soft chined. Having to do with the way the flat bottom encounters and exits the water. As well causing the turning personality to be different in each direction. But I would also suspect that if the boat looks reasonably straight to you, that it will work at least reasonably well. I do not have a scientific analysis behind that, just a speculation.

The more significant problem I believe is accidently curving the boat to one side or the other. Then you will have a boat with a tendancy to constantly turn in one direction, requiring you to constantly compensate for it. That, imo, would be far more annoying. This is also more difficult to visually check, particularly if it's not gross.

Lastly, humpbacking or hogging is a problem, but it's related to speed. The faster you go, the more a problem it is. I doubt it's much of a problem at all with a hand paddled kayak if it occurs in the hull. I have never noticed my slightly hogged kayak acting up like a hogged power boat, no matter how hard I paddle.