Backdraft,

Looking at this picture
http://www.elemodel.com/shopen/index...wt&goodsid=959

My experiance is to do what you did. 1 nut all the way down to the end of the threads, let the molded hex shap in prop hub sit on the first nut. Add a washer and the last nut.

I've never had a nut back off so a lock nut is not required.

Not really a right or wrong thing, just what works.

A washer on the front and back of the prop might allow it to slip and lose power.

HTH YMMV