Every method has benefits and drawbacks. One advantage I see to minh-thanh's method is that copper sheet is more readily available than appropriately sized copper tube, so it's a good method for making copper washers. It's always useful to have more than one arrow in the quiver.
When I said "turn a tube" I meant to actually make the tube from bar stock. It's just so easy to do and you can make washers easily to fit 00-90 screws and smaller.
Another advantage is that the thickness of the washers you make is not limited to the thickness of sheet stock you have. Using the tube method allows you to make any thickness washer.
If Minh-Tranh had access to a hole cutter of the correct size it would not have been necessary to sandwich his material between pieces of wood. When using drills or hole cutters in brass one should grind the lips of the drill or hole cutter to a zero degree rake. If that is not done the drill/cuter will pull itself into brass. It only requires a flat a few thousandths (inch) to modify the drill and it can easily be done with an abrasive disc in a Dremel.
Any way, I figured he was using the tools at hand. If I couldn't find the right size hole cutter I would have done pretty much what he did.
It has been a long time since I pulled a spark plug but as I remember the gasket on a spark plug was not a single thickness.
gmaf - I took that as your meaning, and I have made brass washers that way. I generally have some brass rod in stock so that's easy for me to do. However, I rarely have copper rod around, but I do have copper sheet so Minh-Tranh's method would be a quicker choice for me. It's always good to have options.
john - I think even when using drills ground for brass (or copper), when cutting thin sheet the material still has a tendency to grab and wrap around the drill. The "wood sandwich" method is good way to clamp down the sheet and avoid the hazard of sheet metal whipping around the drill bit.
Drilling any material that is thinner than the point of the drill is a problem. The hole becomes tri-lobed, then the drill punches through and the material climbs the drill. The Unibit style drills work really well on thin material. Asian clones are available dirt cheap.