I was wondering about this as well.  I have a decent clearance along the left Y axis rod.  There are 2 white blocks here, but only the top one lies on the rod, and it has started to get a groove.  After awhile, the rod gathers white dust from this white block wearing down that I vacuum up.

Is this something that I should be concerned about in the medium/long term? Ian, are you twisting the lower block to make contact with the rod?

That's funny. For me it is always the bottom block which always contacts the rod. Mind you I haven't actually used the printer yet.

I tightened the lower block so now there is just a tiny bit of slack between them.

I guess wear on the upper block would explain why the fit between the rods and the lower blocks can tend to loosen.  That would be a good reason for keeping the rods lubricated.

I mean the axis going left and right, the one which moves the extruder itself. I assumed that was called the x-axis.

Yes, your original post said the bed or platform was moving up and down so I thought you were confused.

Is this something that I should be concerned about in the medium/long term? Ian, are you twisting the lower block to make contact with the rod?

I notice there is a significant amount of slack between the white plastic blocks and the X-axis rod. It's a little over 1mm and makes the Edit: extruder tip up and down a considerable amount when I press down on it. Is this normal behavior?