When your complaint is about focus issues...they will likely calibrate the focus. Although unlikely, it may be possible for the camera to be 'within spec' and still not focus accurately with a particular lens that is also 'within spec'.
For example, lets say the tolerance is plus or minus 5. The lens might be +5 and the camera might be -5. The result is a 10 point difference...which is not good.
If you send in both, they can calibrate the camera to the lens or vice versa...or maybe calibrate both. This is good...except that you might have other lenses that do work well with your camera.
I know some photographers that send in their cameras and their lenses. Some do it every year...as maintenance.
I don't know...maybe they won't ask for the lens as well. They only way to find out, is to call and find out.