rather than some dubious wikipedia perhaps this quote from SKF
Precision ABEC 5 and ABEC 7 bearings
Precision ball bearings in tolerance grades ABEC-5 and ABEC-7 are recommended for applications where high speed and/or extreme running accuracy is required.
These bearings have the same nominal external dimensions as equivalent size ABEC-1 grade bearings; however, the running characteristics and external dimensions are held to closer tolerances.
we require these bearings in the motors but not the wheels or transmission, we fit the bearings into moulded plastic wheels and put them through shafts which can in no way be described as precision fits, a higher tolerence bearing is more susceptable to failure due to water /dirt ingress and to keep its tight tolerences requires constant lubrication.
so feel free to spend money on a bearing which will fail sooner due to the application, cost more to replace and offer no performance increase over the abec 1 bearings