The alignment of the z-axis belt in the extrusion channel has always bothered me, so I decided to tackle the problem…
When I removed the z-motor, I found there is wear on the side of the belt. The wear is intermittent. Most belts do not have perfectly-straight edges, and you will likely find areas wider than the belts advertised width, this is normal with inexpensive thin-width belts.
The alignment of the belt will interfere with the accuracy of the z-axis movement, how much is extremely difficult to measure. Belts like these are designed to run true and square to the gears, if they are not running true then backlash is definitely happening, especially true of thin-width belts.
I found the squareness of where the extrusion meets the z-motor is not square, structurally it’s not required to be square. However, the squareness could potentially effect the alignment of the z-axis belt, so I milled the end of the extrusion to be square… the alignment did not visually change…oh crap. Well, must be the motor mount…nope…aha…it’s the screw-threads into the extrusion. It’s a compounding effect, the ribs of the motor are not level with the frame of the motor, the stepper-motor exterior is not designed to be touching anything other than a mounting-device to the nemaXX mounting-frame and optional heatsinks.
I’ve made a custom shim to fit the profile of the aluminum motor-mount and now my belt runs true and square to the gear. The shim also provides adjustment for the belt tension, a belt without proper tension will introduce backlash into the z-axis movements thereby interfering with the intended precision of retraction z-lift, layer-height, layer-width and nozzle-height calibration.
edit: Tiertime have provided you with an affordable 3D printer with exceptional value and great print-quality “out-of-the-box”, there’s no-way I would ever say/feel anything negative towards their Cetus3D product(other than warranty-support, but only if it ever dipped below a certain standard). There’s always a cost balancing-act that takes place with any commercial product, be it engineering costs, assembly costs, quality-control costs etc etc…the list is long and labor costs play a huge part in the production processes. Yes, the printer can be improved, Tiertime knows this and encourages modding… if …you want to take the printer to another level 
Cheers,
Brent