Quote Originally Posted by CodyPilla View Post
Is it necessary to get official recommendation first?
I believe that you need to fill in the forms in triplicate and make formal application to Hiwin before fitting any of their profile rails...

More seriously, I can remember finding a copy of the Hiwin manual giving their recommendations for fitting and setting up their profile rails when I built my router. As M_c says, 20mm rails are intrinsically stiffer so easier to mount, and particularly for the Z axis the increased height over smaller rails makes it easier to fit in the ballnut, etc, between the Z fixed and moving plates.