I have a couple different Tanaka gyutos. The spine is often not perfectly straight. On one the cladding is heavier on one side by the handle. On another, there is a slight twist to the entire blade (very slight) With a clad knife (more so, iron) there is some give and take, and you may be able to torque it a bit more straight by hand. I would say, if you don't notice it in use, or just by looking at it without close inspection, just accept it.