The washer being smaller then the penny had a better look to them the base was much smaller and look better proportional to the figure. basically it just looked right to me. I know pennies are cheaper and washers cost more for a box of 100 they cost about $4 dollars for 1/4 inch size that's the hole diameter.
A bonus is that you can line the bottom of your storage box with magnetic strips to keep your figures in place while move such boxes around with very little to no harm to your figures.
Be sure to wash your figures first to remove all the mold release agents before you do any work with your figures.
I like to use wax paper when mounting the figures to the base, I lay the wax paper down layout my washers the add Super Glue Gel to the washer then place my figure in place. the wax paper works well to prevent the washer and glue from sticking to any surface that you really don't want it to be glued too.
Now I can build up the base for it to look right. I like a smoother look to my bases just seems right to have a nice looking smooth base other then rocks and plants on the base when you have your figures running around an spaceship or building and the have plants all around their feet. where as when they are on open ground is not as noticeable.