Yeh bud, if your milling a datum edge in for locating the rails then the machine your using to mill the datum with will have to be cock-on!

Regarding the blocks you always mount the rails like this and let the blocks find there own parallelism. So for instance when assembling a z axis mount one rail up agains the datum edge then mount the second, as for when you aline the second rail to the first; mount a bearing block on the first rail thats correctly mounted the attach a magnetic block and gauge to clock the second rail! This is a method of checking if you have milled both rails a datum edge or a way of aligning if you are using a single datum edge. then mount all the blocks and attach the second plate then attach a gauge to this and align this plate to either the first rail or the first plate (taking in consideration that the plates have good milled edges.)