Application of bearings
Long ago, roller bearings ruled. A cone on a threaded shaft on each side of the trucks. Bearings in the races of the wheel and another cone on the outside. A d washer to promote locking of the axle nut. Setting the looseness was up to the skater. 2 cones, 2 races per wheel. Self aligning. Tighten too much, wear out the bearings, too loose, lots of noise and maybe loose your bearings.
Precision bearings. Self contained, light weight, easy to install. Older wheel with out hubs, tighten too much, bog you down, loosen the nyloc up and wheels run well. However along that time came erethane wheels that were pretty soft so from the deformation of the urethane the side loading would allow the bearing bores wall the squish into the bearings covers and cause drag. So the spacer was put there to control the distortion of the wheel, keeping the wheel bearing bores from getting into the shields.
Hubbed wheels. Accept the precision bearing with or without spacers. Some bores very tight, some quite loose. Much better grade of wheels than previous types. Outdoors other types can be better than hubbed.
Precision Bearings. Very precise. Tested thoroughly for decades. I tore down a blower motor the other day to attempt to repair it. Lol, 608 double shield bearings! It had ran for a couple of years, no grease fitting to lube it. Replaced the bearing, motor ran again. 2 bearings on a common shaft, one end fixed (not moveable), one floating(bearing can move a small amount if needed to prevent race binding). Nothing wrong with 608 bearings, grease them, align them, give them room to prevent binding and that is as good as it gets. Differences in bearing grade doesn't change speed enough to overcome a difference in wheel performance.
Installing spacers will align inner races, axle shaft does the same thing. So the hubs align the outer races. So bearings are aligned as well. The only thing in question is spacing. Tighten the nylocs too tight and the bearing side load the races slowing the roll. Bearings are aligned but bearing races are sideloaded. Wearing out the bearings. Nylocs set at 1/2 turn backed out from snug, bearings run correctly in the grooves/races. Bearings still aligned.
So alignment isn't an issue except if your axles shoulders are too short.
so, alignment, not an issue. Sideloading not an issue, without spacer, run 1/2 turn back from snug. Spacers, fit each spacer to each wheel with a pair of bearings x 8. Possible to find you have too narrow of spacer as the wheel hub bearing bores are not as precise as the bearings machining tolerances. Usually spacers are on the slightly long side... wonder why that is? :)
So if a pair of bearings are running true in the races with a proper length spacer or without spacer they should perform the same. No better, no worse. However, during push or cornering, you are actually angle loading only one bearing if without spacer in the wheel. With spacer you angle load both bearings.
So the differences are minute, bearing spacers or not. Lol now talk to me about flip axles.............
Pick crappy wheels, you will wipe out any possible gains of bearing choice or spacers many times over.
Slight differences in wheels performance are greater than any difference in good bearing performance(unless it's worn out and rusted crapola).
About them flip axles.....