Where exactly are you fitting this shim?
My front caster wheel setup is s follows:
Countersunk cap screw, Flat/countersunk washer, fork leg1, carefully machined correct length alloy spacer1, bearing, another carefully machined alloy centre spacer, bearing 2, another carefully machined spacer2, fork leg2, another flat/countersunk washer, another countersunk bolt.
If any of these spacers are the wrong length then the wheel will not be central, or the bearings will bind, or the axle will not pass through the fork leg far enough (or too far) etc. You just need to built accurately and with care.
Stock axles do not go through fork leg but rest still applies.