Just an update on this post. I emailed CPR last night and recieved a reply already. I love Good service! Kevin advises that:

"Storing them standing on end (leaning them against a wall) tends to make them "collapse" toward the wall.  I assume it was face-out from the wall?  The corollary would be leaning it against the wall with the artwork toward the wall... it would bow (eventually) in the opposite direction (the center lifted, with the ends down).

No matter, as the bow is always slight.  Once installed, the rails bring it back to level.  That is the function of the rails screwed on all around the outside.  So it will be moot when swapping.  You'll be fine.

For future storage, always let the playfield rest flat on its back."

Probably makes sense. I have laid it flat in the meantime with some records on top ( good use for my wifes Melissa Etheridge albums!).