Axle Washers
Axle Washers (Also known as Axle Spacers, Speed Washers or Speed Rings) are small metal rings that you slide onto your truck axles to encapsulate the bearings between the hanger and axle nut. These are meant to create a smoother ride and help prolong bearing life. You put one axle washer between the hanger and inside bearing, and another one between the outside bearing and the axle nut. This helps reduce friction, limit slop and keep your bearings from binding up. Without axle washers your bearings can end up rubbing against the truck hanger and axle nut.
Freestylers also use axle washers to adjust axle depth and where the Wheels sit in relation to the edge of the Deck. By putting multiple axle washers behind the wheel, you can push your wheels outward, bringing the axle further inside the wheel (to avoid axle contact with the pavement), or to adjust the wheel-span for optimal rail alignment.
With regards to wheel-span, the wheels should never extend past the edge of the board. Placing the wheels even with the board will, however, help with Rail Whips because it lessens deck drag. Bob Loftin recommends an optimal wheel-span somewhere around 1/8" to 1/4" narrower than the Deck. It is all a matter of preference, but this range can provide better rail stability. Just try some different settings and see which configuration works best for you.