A lot of kits have the right thrust built into the kit. The left fuselage is just about 1/8" longer than the right. If the plans don't call for it, then I would leave it out unless a taxi run proves that you need it. If your kit does need it I found that 3 washers works pretty well. I don't like the idea of the wedge pieces because they could come out due to vibration and then your motor mount would be loose!