On page 9 of the current instructions (April version?), for the x-axis carriage plate (Assembly #3):
Regarding part number 13 (the two silver colored custom extrusions held on to the back of the plate with square nuts that engage in slots -- strangely, they are listed in the parts but not identified in the drawing...). It is important that these be parallel to the vertical axis of the plate, equidistant to each other at their tops and bottoms, and are pushed towards each other given the slop allowed with the v-groove wheels.
If they are not parallel to the plate, your router will not be perpendicular to the table. (This I learned the hard way.) [By the way, here's how I figured that out: remove the router, make sure the power is off so you can move the carriages freely, and slide the z-axis all the way down to the waste board. Look where the x-axis carriage plate is touching the waste board -- it should be contacting the waste board evenly.]
If they are not equidistant to each other top and bottom, the carriage will get looser / tighter against the extrusions as it goes up and down. In other words, your router will have more slop depending on whether you are at the top or bottom of its range.
Lastly, if the extrusions are too far apart from each other, you may not be able to adjust the offset washers enough to get a nice firm contact between the v-wheels and the extrusions.
They can be adjusted while the machine is assembled (I just did), but its not fun.
And in order to tighten the bottom set of v-wheels, you'll need to slide the y-axis carriage all the way to the front so that the z-axis hangs off the front so you can lower it just past the waste board and get to the hex nut from the back.