the skewer
always compresses the axle and preloads the bearing (if it was set to have no clearance) or removes the last bit of free play (if the bearings were correctly adjusted, i.e. so that there was a tiny bit of free play without the skewer fully tight).
With traditional cup and cone hubs it is possible to use a special tool ( 'hub vice') (see Sheldon Brown) to simulate the QR loads during adjustment, and thus get the hubs set up 'perfectly' i.e. so that there is
just no free play with the locknut tight and the QR pressure applied.
Sheldon Brown on hub adjustment
https://www.sheldonbrown.com/cones/index.html including details of his special tool made from an old cone and short length of axle
Jobst Brandt on hub adjustment
https://www.sheldonbrown.com/brandt/cone-adjustment.htmlFWIW the typical compression of a steel axle is ~80um; (the rear is typically longer and fatter (10mm) than the front (9mm); the result is that both compress about the same amount). You can use this to get very close to the correct adjustment; just back off the cone 1/12th of a turn (if the axle has a 1mm pitch thread) from the adjustment that (with the locknuts tight and no QR pressure applied)
just gives a play-free adjustment.
cheers