Originally Posted By: Vikas
This makes sense; doing it this way will flatten the washer "more" correctly. Actually, this is the way it is prescribed by Honda on my lawnmower blades. The washer is indeed concave.
On most oil plug washers and most washers in general are not concave like your mower blade analogy. If understand the OP correctly, he is talking about the outside edge of one side of the washer being slightly round but still has a overall flat side. The rounded edge on most washers is inconsequential in most applications and it if was design critical or orientation, it would be annotated as such on any maintenance manual literature.
I am familiar with those concave washers on mower blades and they are design to apply tension to both hold the blade in better due to usually being over sized and also act as a lock washer function to keep the bolt from backing out.