A quick question if I may.
http://www.valuframe.co.uk/Technical-details.html

The Valuframe profiles have a 0.1mm camber either side of the profile slot. To my mind, mounting a profiled rail to this slot slightly to the right or left of centre may result in the rail ending up rotated with respect to the plane or possibly twisting slightly along its length.

A) Does this in fact happen?
B) Is it a significant effect?