Wood is composed of fibers. The direction these fibers are running in, can be called the grain of the wood. There is no such thing as "a specific grain line" since grain does not follow lines. You are probably talking about (annular)
growth rings. Following a specific growth ring means you are following the grain in the radial plane, but not necessarily in the tangential plane.You ALWAYS want to follow the grain of the wood as much as possible in bowyery, but some grain violation is inevitable.
Clear as mud, right? Yeah, that happens when you ask a vague question!