If built per the kit instructions, it should be nice and stable. When assembling the rear uprights, you first slip two 0.5mm plastic spacers over the axle. Then you slip one bearing over the axle. Then you press the upright over the bearing. Then you slip a 4mm plastic spacer over the axle. Then you press another wheel bearing on. Then you install the blue tapered washer onto the axle and slide the pin through. Now the wheel slips over the pin. Once you tighten the wheel nut down, the back of the rim is pressed flat against that blue washer. This in turn squeezes every little piece together. If anything is out of order (or missing), it won't work right.
If your experiencing play between the wheel bearings and the upright, then you can always try applying a thin layer of CA glue to the inside of the upright where the bearings press in. Once dry, it should make for a very tight fit when the bearing is pressed in. You just don't want to use too much glue or it will pool to one side and your bearing won't sit centered. I've built two of these cars, and I can say you shouldn't have to do this...
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