Bicycle Mechanics - cones "settle in" (loosen) in newly-overhauled hub?

I just built up a singlespeed rear wheel for someone on the west coast and am going to ship it to him. The wheel in question is an old freewheel rear hub, with different spacers so as to get the singlespeed freewheel at 42.5mm chainline. There's also a new (solid) axle, but the cones are the original - used though in very good condition.
I'm currently using a mishmash of steel spacers - washers and nuts - that make up 6mm of spacing on each side. But I'd like to use aluminum 6mm spacers, which would be simpler and more elegant.

Here's what I'm wondering. I've got the hub perfectly adjusted, but I would like it to be perfectly adjusted once the guy starts riding it. I think that my mishmash of steel spacers aren't going to "settle in" at all with riding - but I'm worried ant 6mm aluminum spacers might settle in a bit, and compress just a bit, which would mean that the hub ends up slightly loose instead of perfectly adjusted.

I don't think if this is something that happens - does it? Thanks.

Aren't those spacers outside the cones? If they compressed it would mean the stays/dropouts would be pushed a slight amount closer, but how would it affect the cones? On a quick release wheel, the axle itself can apparently be compressed, so that affects the cones slightly, but this is a solid axle so that shouldn't happen.

What condition are all of the components?

If you were building a hub with all new components I wouldn't worry.

Pitted cones won't adjust very smoothly. Sometimes locknuts won't stay tight on worn axles. If you rebuilt using really old bearings that had been run dry for awhile, they may be ovalized and will reorient themselves which will result in the hub loosening.

If you rebuilt using really old bearings that had been run dry for awhile, they may be ovalized and will reorient themselves which will result in the hub loosening.
I think this is an old-wives tale. Bearing balls rotate on all axis as the wheel spins so they don't develop a non-spherical profile. Bicycling magazine's tech editor used to repeat this story until an engineer from one of the big bearing manufacturers wrote in to discredit this theory.

That said, I replace the balls with every annual hub rebuild anyway. :)

I think if you get the hub adjusted proiperly and cinch the lock nuts down HARD on the cones, you will remove any slack or burrs and the hub adjustment should hold.

Retro Grouch: actually I assume used components wouldn't "settle" if there is any of such to be done.

The locknuts are cinched down hard on the cones, although there are spacers in between. I figured that hard tightining the of the locknuts with respect to the spacers would prevent any over-time compression of aluminum spacers; I only worry about it because the aluminum ain't the hardest metal.

I'm with HillRider on the bearings wearing non-spherically. I guess in some environments where bearings never change orientation this might happen, but not in any cup-and-cone environment, and also unlikely in most sealed cartridge setups.

why not ride it with an old tire for a while before shipping it?

I think this is an old-wives tale. Bearing balls rotate on all axis as the wheel spins so they don't develop a non-spherical profile. Bicycling magazine's tech editor used to repeat this story until an engineer from one of the big bearing manufacturers wrote in to discredit this theory.

I kind of expected that somebody would dispute that one, and I'm glad that it came from somebody who I respect, but I'm not quite ready to give it up yet. It's been awhile but I've used old parts to overhaul some crappy old dry hubs and bottom brackets which needed to be "run in" and readjusted before they would stay tight. It's got to be either ovalized bearings or operator error. Since it's happened to enough different people to have become an old-wives tale, I'll stick with the ovalized bearings theory and avoid the operator error stigma.