If the bearings say the ABEC grade, and they are from a reputable supplier, then they will be that grade. Since we use a pratfull of bearings compared to the rest of industry, it would be an idiot who tried to supply a bearing not of that quality, and here's why...
The dimensions that are specified will always leave a clearance between the balls and the races. In most applications, the bearing is designed to work best when the clearance approaches zero. Additionally, in order to keep bearings in position, they usually have a press fit into their housing, or onto their shaft. That press fit is determined by the requirement of the shaft or housing, and its tolerances. In order to get the bearings to end up with no clearance between balls and races - the best running condition - you have to select the bearing you need by its ABEC number, or clearances, and the duty the bearing must support for the application - speed, load, etc.
With a high ABEC number, the interference of the press fit cannot be too big otherwise the outer race will contract (or the inner expand) too much, and the balls will crowd into the race reducing life and performance. If you need a large interference fit to hold the race, you have to have a lower ABEC number so the contraction of the outer race (or expansion of the inner) when it is pressed on the housing/shaft do not crowd the balls into the races.
The problem in model car terms is that we have almost no control over the size of the shafts (in metal) we use, and certainly no control over the size of the (plastic or metal) housings. When I say control, I am talking about shafts in things like power tools (50k to 5k spin speeds, close to our speeds) that are ground to tolerances of 0.0002" (about 0,005 mm) in order to get the correct press fit and not crowd the races.
You can spend any amount of money you like on your bearings, have them at any ABEC grade you want. But, and it's a big but, if you press an ABEC 7 bearing into a housing and it is a tight fit, there's every chance the outer race will contract to the point where the balls are crowded (pushed) into the races, and the bearing will last very little time at all. Similarly, you could buy a cheaper ABEC 3 bearing, press it into a housing and have perfect ball/race clearance and the bearing will last an age.
Far more important than ABEC rating is the duty of the bearing you buy. Is it rated for the load and speed you are asking it to take? And where do you find that information from any RC bearing supplier - I've never seen it!! Even if you have the right duty bearing, you then have to know, very accurately, what size shaft and housing you are putting it into, and what effect that will have on the final clearance between the balls and the races. If it's loose then the performance will be lower, and if it is crowded the life will be short.
If it's any use to anyone, select your bearings by the quality of the materials (stainless steel races, ceramic coated balls, silicon nitride balls, etc.) and the quality of the shields for the application. The best bearings ever made for RC cars were the blue-seal Tamiya ones of the '80s and '90s, although I've seen some good copies of those recently. Not only were they the highest quality of seals, they were high quality materials too.
Not getting at anyone here, but frankly if you buy an SKF, NSK, Timken... (see Andy110M's list!) bearing at ABEC 5 that is what you are getting. Buy no-name Chinese and you can't be sure. And if you buy ABEC 7 and put it into a tight housing, or have to push it hard onto a shaft (or worse, both!) chances are you will crowd the race into the balls and the bearing won't last long. ABEC ratings are one thing, but they are a long way from being the only thing, and I would argue they are not worth a candle for our applications - I'd rather have good materials and the right shield to get my car through the racing calendar, and take pot luck with the clearances in the hope that I haven't crowded the races and shortened its life - it's all one can do. HTH
