What kind of stringing is incorrect and what is normal?
The question in the title is a bit provocative and certainly rhetorical and applies to 3D printing in general.
I understand that there are no absolute answers, but I want to somehow calibrate my own thinking. I want to avoid unnecessary precision and stay within reasonable limits with my criticality.In other words, what is normal in 3D printing and what is over engineering?
In the picture you can see how my very first print came out after assembling the kit. I haven't made any changes to it, rather it has been run as it was on the manufacturer's USB stick. (Click on the picture to see it in full)
I must say that I am surprised by the high quality of the kit and the easy assembly. Thanks for the good instructions, this community and of course @iftibashir for the great videos. The only fine adjustment I made was that I sacrificed one feeler gauge leaf under the Y-axis motor mount, because the angle of the pulley due to the thermalpad was so bad that I couldn't leave it like that.
You need to remember the purpose of the sample files on the USB stick: They are to ensure the new user has something that is almost certain to print reasonably OK on a new, not yet run-in or well calibrated machine. The example you have shown is extremely easy to print but sacrifices quality to ensure this.
Your precision limits are about half an extrusion width - so with a 0.4mm nozzle extruding 0.45mm wide you should expect 0.2mm repeatable tolerance. You need to squash the layers together for adhesion so 0.2mm layers, perhaps 0.15mm, are about right for most work but the extrusion 'width' in the Z direction is still 0.45mm so the vertical tolerance is also about 0.2mm. With a couple of years experience you will be able to go tighter but leave it for now.
Stringing? With properly dried filament PLA should have none, PETG a few whisps, and FLEX may need a little work to clean up. Other filaments may be trickier.
Cheerio,