Are washers necessary?

I'm building a wood and fabric plane that uses plenty of aluminum
fittings and
hinges. If I have an aluminum block on one side of a spar made of wood
and the aluminum hinge on the other, with a bolt going through, is
there
any reason for washers? It's not that I wouldn't use them, but it
helps when
ordering the correct size bolt. The plans don't mention washers but
the plans
don't call for a few items that seem to me to be necessary.
Lou