Aligning the rails isn't the only thing to consider as a temperature change, particularly if you use aluminum mounting, will cause a change in distance due to thermal expansion effects. So, even if you set it up perfectly with very little force required to move the blocks if the temp changes by even a few degrees it is possible the relative distance will change enough to apply side forces of tens, hundreds or even thousands of newtons. Again, the typical way to deal with that is to have one block rigidly mounted to moving table with the second block permitted a degree of freedom side-to-side so that even with substantial change in temperature there is no significant increase in side forces. The second block should be used to confine the motion of the table vertically and fore/aft but allowing that side of the table to expand/contract with temp change.
Brian