to expand a little on speedy post the outer rim/casing of the bearing is held on its inside lip by the wheel but the smaller bearing ring is being pushed from the out side by the nuts/hanger and this makes for a funny load if you tighten them up that pinch and deform the bearing path.

If you run with a little bit of slop then your loosing performance as the wheel and bearings are not being held square. This leads to chatter and the bearing pinching when sliding and hard cornering. Also with the bearing not being held square top straight line performance is slightly reduced.

Use the correct size (0.400inch - 10.16mm for most wheels and 0.300inch - 7.62mm for some kryto and a few other wheels) but the incorrect size is not much better than than no spacer at all.

The reason spacers works is to hold a solid line of metal from:

axle nut -speed ring - bearing - spacer - second bearing - speed ring - hanger.

This solid link is often found in large machinery with high rpm or big loads so to me the best I can do for my skateboard! It has been a long time since I started using spacers but recently found my self short a set and was quite surprised how much of a difference it did make, I felt slightly slower but when cornering I felt that my exit speed was lower and my usually rather smooth sliding.