The carriages have an adjustment in the bearings that allow them to be snugged against the steel, and even pre-loaded a little bit. Cold rolled steel is remarkably accurate from the mill. It is spec'd at +0 / -0.004", and that's between different pieces of steel. Within a single piece (which you adjust the carriages to), deviations in the 1/4" dimension are often less than 0.001".
This z axis is the one used in the 2' x 3' plans (and upcoming 2' x 4' R&P plans) from cncrouterparts and fine line automation, and has proved itself on many different machines -- check out the build log section of my website, and you'll see a lot of them!
Hope that helps,
Ahren
www.cncrouterparts.com