I don't believe there is any performance gains by doing so. It's only done on motors where the piston would hit the spark plug, like on a big block chevy with hi compression.

And when I raced bb chevys I rarely used the washers, just found the right plug for the hole. You will need to start with 3 sets to get a good set. When I did use the washers I didn't remove the factory washer first, It don't matter which way you use the copper washer, it'll seal either way.