The fabric reinforced belts that come with the printer are OK, to a point.
There are loads of belt tensioners and frame reinforcement modifications - use some, I particularly like the Y tensioner frame, this one https://www.thingiverse.com/thing:2045010, and for X, this one. https://www.thingiverse.com/thing:1683070
If you look on Thingiverse there are several threads discussion a multitude of possible causes for non circular prints, but belts cause non circular prints and should be the first thing to check, not as in my case, the last. Of course its always the last thing you check that is the fault you need to find.
Speaking from bitter experience, AVOID steel wire reinforced belts, yeah they don't stretch, but the wires DO BREAK because the GT2 sprockets are so small, then they stretch and cause no end of problems.
Mine broke and there were fine wires sticking out of the smooth back. If your belts stretch you will finish up with non circular prints and not in the direction you expect - which makes fault finding difficult.
Tension the belts with care and absolutely better loose than too tight - If your prints aren't perfectly circular: