Yes, if the washers are not correct, the bolts could bottom out and you would not get a good seal. Maybe that's the case; but I don't know enough about that engine to tell whether they are the correct washers.
You can subscribe to the Mercedes Electronic Parts Catalog for free and input your VIN number to get the exact part number for the washers and the bolts.
EPC-net Online
It will even store your VIN for later use. Even if you decide to go outside the factory parts, having the factory part number to insert on the search minimizes error. Being able to print out the diagram is nice too.
On the M119 engine, there is no way to lay the valve cover on a flat surface. Maybe on yours you can.