I think I will do what others have and add a support track and bearing at the far end of the X-axis. After a lot of frustration I got the tube level and did a dry run for my first project. I then left town for 3 weeks. When I came home the X-axis arm had sagged 3/16" at the last slat. I know the 8 screws are tight so there must be enough flex in the Y-axis tube to allow the sagging.
The tube steel will not flex. The gantry bolts hold the x axis in place so any movement is usually here. One other location of possible flex is if the y axis plate bolts aren’t tight enough holding the upper and lower y steel tubes.
But yes, if an issue, adding a roller of some sort on the end tip of the x axis will remove any vertical sag.
I have the XL tubes on the y axis and taller plates I cut with the table to add extra space for cutting thicker stuff like tube steel. With all that, I do not have sag and the THC Z axis is installed.
Do what fits your needs best.
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You might consider taking each stanchion plate loose and before re-assembling, coat flat surfaces, where the stanchion plates will contact the tube, with this product:
It helps to add friction between components. It is meant for smooth shafts and bearing collars.
And, certainly, use the blue thread locker with all of the bolts that hold the stanchion plates.
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I can not believe that someone has not commented on their Sagging X yet. 
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Problem solved. Because I got the extension kit I used to standard rail for the track with a bearing. Variation is not much more than 1/32" over the whole table.
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Nice job. Does it roll smoothly? Great idea.
Very smooth.
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