Created attachment 213518 [details]
Maximized GIMP window with position saved to disk.  Upon restarting, the interior window area is slightly offset due to the system drawing borders around the window itself.

Yes, this is actually a bug.

The following steps were performed when running GIMP 2.8.0-RC1 in single-window mode on Win32 XP:

1 - Start GIMP and maximize the main image window.
2 - Save window positions and exit GIMP.
3 - Restart GIMP.

Notice that the window is not maximized, and furthermore has been slightly offset.  This occurs because a maximized window may or may not incorporate the border decorations typical of normal (non maximized) windows.  As a result, when GIMP sizes and places a (non-maximized) window according to the (maximized) width/height values in sessionrc, the actual position of the window's interior area is offset by whatever border decorations are provided by the user's system.  This causes the right and bottom edges to extend past the edge of the user's actual desktop, resulting in a cropped window.

Illustration is attached.  In my case the difference is only four pixels wide (noticeable but not serious), but if a user has their system's border decorations configured thick enough, this could cause tools/options near the edge of the window to be completely shoved offscreen.