The fiber washer is to protect the porcelain from chipping out as bolt pressure comes to bear.
It is a cushion.
So to place the fiber washer on top of the leg would do no good.
The most vulnerable place would be all the bolt heads that hold the grate supports.
You have higher "point-loading" here and almost no "cover", so if you chip out, it's very likely going to show.
Edit:
Don't go full retard on the bolt torque.
Snug 'em up and yer done.