Unwashed it will cause your water to turn milk white for a few days. If you have corals & fish in the tank you should thus wash it (especially if you plan on adding a large volume of sand).
If you are starting a new tank (or if you can remove the inhabitants for a week or two) it would be better to add it unwashed, as the very fine dust is actually tremendously beneficial to your DLSB - it just needs some time to sink to the bottom (anyone heard of Miracle Mud...). Also, after this period, you should not disturb the surface of the sand bed for another few weeks, until the 'mud' has been worked into the sand bed, and/or become covered with a bio-film (bacteria and algae) - after that, the water should not become cloudy even if the sand is disturbed.
Hennie