Why is it important to keep bearing sets when greasing a campagnolo hub? In almost all cases where hubs are regularly serviced, bearings don't need to be replaced. The general consensus though is to replace bearings after so many thousands of kms. Many posts on the web say a grade 25 bearing is fine to use in a hub. 

I have a Campagnolo Record hub with loose bearings (not in retainers). The races and cones are fine and yet the wheel drags slightly at points. Backing off the tension leaves the hub very slightly loose and mostly solves the courseness of the hub. This is what I expect with normal wheels, but not a Record. Why should my Record hub need some slack when the races are precision ground and hardened, and Campagnolo selects bearings to within 1 micron, or about grade 25?

Did the other people who serviced this hub in the past use just any general bearing? New bearings have been introduced by many bearing manufacturers as high efficiency, where they select bearings to within 1 micron. But, in a bicycle hub, it's not so much a matter of efficiency. It's about smoothness and road feel, and loose-ish hubs just don't deliver that.

It seems the answer to my problem now is to either buy a 9+9 set of matched Campagnolo hub bearings or to buy grade 3 Si3N4 ceramic bearings which are manufactured from the same lot. Even better would be if I could size my own bearings to within 1 micron.

Any other suggestions or ideas?

EDIT --------

Just to clarify. When I talk about drag from this hub, I mean when I spin the wheel and rest the axles on my index fingers I can feel and see some movement. On the other hand with a Mavic 500 with sealed cartridge bearings I can lightly tension the bearings and feel no movement when spinning the wheel.

With either wheel mounted on the bike there's no play (without load). So, when I say I backed off the tension of the Record hub it's not something you would normally notice in a setup for a bike, but I've found in riding I prefer my hubs set a little tight. Maybe it's because I tour and carry an extra load on the rear. I tend to push my cornering hard on downhill switchbacks and I find tighter hubs give me more of a sense of hugging the road.
In almost all cases when servicing a hub the bearing DO need to be replaced.

You may not be able to see it without the aid of a microscope, but the bearings will be slightly pitted after any appreciable period of use. As one user points out, high grade ballbearings in case hardened steel, even grade 25s are very cheap - why cut the corner?

Case hardened are preferred as they are less brittle, BTW.

Bear in mind that in correct bearing design, as it's difficult to get an exactly consistent rate of wear on the three components, you will generally find that the balls (being the cheapest and easiest to replace part in the assembly) will wear first. Then the cones. Last will be the internal bearing surfaces (in the case of the hub) as these are the most difficult to replace. Cones and cup wear rates may be the other way around if the cone is integrated with another component or part of the assembly that is difficult to access or change - not normally the case in any conventional cycle application.