Do you mean the top hat washer that fits in the Spider or Chainwheel/sprocket to fill in the huge hole in't middle to make it fit the crank?
On a 1 pc crank you can fit them either way because the retaining lock on the sprocket side of the arm is usually long enough to reach the Chainring/spider/sprocket.
Normally the top hat washer is on the inside hidden by the chainwheel/spider but if you can't align the Chainwheel/Sprocket/Spider to the freewheel then you might want it on the outside to move the sprocket/chainwheel/spider closer to the BB to get alignment with the teeth on the freewheel.
Make sure your chainwheel and crank arms clear your frame because they can flex and cause damage to your frame if the clearance is too small.
On 3 pc Cranks the Top hat washer should be placed on the inside (hidden) because the retaining/Sprocket bolt needs to tighten and fix the sprocket/spider to the Drive side crank arm. If the top hat washer was on the outside then when you tighten the retaining bolt it will pull the sprocket/chainwheel/spider out of alignment towards the crank arm and cause it to go skewed!!
It will then wobble when rotated and could cause the chain to jump off under duress!!
