Most will agree that my flat washer views are extremely trivial but here it goes anyhow.
Most of the time this issue is only for aesthetics but for fixtures there is a practical side. Most common, if not all, flat washers are mass produced by stamping them out of sheets of metal. Stamping is a shearing operation and because of the plasticity property of the metal it will become distorted. So, there is a top and a bottom to a flat washer.
One side of the washer will have rounded edges, I call this the top. The other side will have sharp edges, I call this the bottom. For aesthetic reasons, if the washer is in plain view, it is nice to have all washers with their top up or in other words the top exposed. The rounded edge is produced when the shear die first hits the sheet metal and smears the metal toward the bottom.
Photo below is a close up of the rounded edge:
Photo below is the sharp bottom edge:
The sharp bottom edge is that way because of the continued plasticity flow of the metal. The practical aspect of this is when fixtures are used and frequently being fastened and unfastened with bolts and nuts. Especially when using hardened steel washers. The washer will have a tendency to rotate and if the sharp edge is facing your fixture damage can result to the fixture.
Bonus photo of the shear lap showing through the zinc (or what ever) plating:
Oh well, so much for my slant on Mr. Washer.