That wont work too good.. the Hex's need direct contact for efficient drive...
Your hex has a pin that secures it into position.. putting washers in it means the pin wont fit into the hex and then the hex wont spin with the shaft making it useless.
HPI make 2mm of set spacers with some of their rims and is a better alterative to your washer approach..




My Escalade on a HPI Sprint 2 and the wheels are 0 offset.. I usually use 6mm on the rear and 3mm on the front.