If I were you, I would try a real world test of the "no play" expectation. Bolt down one of your rails rigidly. Then bolt something long and rigid (thick bar, angle, whatever) to the slide block perpendicular the the rail motion. See how much play really exists when you rock it back and forth. A lot of people that build machines worry about even 1 or 2 thousandths backlash in their screw/nut/bearing combination of the drive system. Any "play" that you get from the rocking slide block will be considered backlash and will be added to whatever (if any) backlash you have in the perpendicular axis' drive components.