On toy trains with all geared wheels, the quartering does not have to nearly as precise as scale models. With all geared wheels, the side rods carry no load and are just along for the ride, so if they are out of quarter, it is a visual effect. You can file the holes in the side rods a little sloppy and create a situation when they don't bind.
On a model with a gearbox on one axles and then that axle transfers power to the other axles through the side rods, quartering is critical to make the loco run and run smoothly. Most brass steamers and high end steam models are built with this kind of mechanism. A quartering jig which most of do not have at home is pretty much needed to do this if the wheel is already loose. If you are about to loosen it and it is in quarter, you can scribe the axle and wheel center a little and remount matching the scribe marks.