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.
im thinking that it might be a bit of a pain if your using 15mm rails and a little less so with 25mm rails... either way make sure you buy the groved slot nuts and your rail should sit pretty close to central then do your best to centralise before swinging on the cap heads ... the camber is suposed to act as a locking device and will pull out under tension so if your on 25mm rails there shouldnt be any twist at all