5mm of slack or slop?
No, this is not normal. I would also attribute this more to 'tear' (or abuse), than wear.
Bellows rack should be rock solid, even at full extension.
People don't fully understand how badly they are abusing their RB's & RZ's when attaching longer lenses, without using the extension support brackets; 100-200mm Zoom, 350APO/360, and both 500mm lenses.
That's possibly the source of your woes.
Even when using extension tubes, care must be taken that the bellows-rack system is not over stressed.
Using a bread-board or laptop table type support for long lenses, or lens extension tubes combinations will help in keeping your bellows-rack system in good condition, or aid an ailing rack system like you have described. Mount the camera to the board, off center, then use bean bags to help support the weight of your lens/tube combination on the other side of center.
A few ideas attached.
http://www.ikeahackers.net/2012/02/daves-lowhigh-rider-laptop-table-hack.html
http://www.instructables.com/id/LAPTOP-DESK-FROM-OLD-CAMERA-TRIPOD/