Definetly use plates. Using washers or nuts is not only illegal, but very unsafe. The wheels need to have a flat surface to mate to. washers and nuts won't provide this, and will cause incorrect torque on the lug nuts.
on thing I will mention if you use plated, make sure that the stud length is sufficient. You may need to install longer wheel studs to maintain proper thread engagement, you wan;'t to have at least 1 1/2 times the stud diameter worth of threads to engage the nuts for proper retention of the wheels.

Bill Burmeister