[Zorbit]

The "buckling" stiffness really determines how worthwhile a higher precision/stiffer(wider,preloaded) rail is.

Because of the substantial load carried by the rails this does provide some of the benefits of "preloading"to the x and y axis(but not the z)Especially on the longest axis.
Medium preload=1/3maximumload,newtons/10=kg). Would this be enough?Preload is applied to all bearings all the time of course,whereas gravity doesnt achieve this.

Good points, also worth considering whether heavy preloaded carriages can operate at sustained high speed used on some routers without overheating either themselves or the motors driving them. I assume that heavy preloads shorten service life, as they are working all the time.