I've noticed that the wear on my tire rim is uneven. The black indicator groove is still visible in some places, but worn down in other places. Both the left and right side braking surfaces have this uneven wear, and, where one side is worn down (indicator line no longer visible), the opposite side still has the line visible. I thought that the wheel might be out of true, but after inspecting the wheel the best I could, that doesn't seem to be the case, and the wheel doesn't have any noticeable wobble.
Is this how braking surfaces on rims normally start to wear out, or does it indicate a problem? Does anyone have any thoughts on whether it is time to replace the rim/wheel?