No need to wash...you can soak them in warm water if you feel the need, don't use any soap or shampoo. What are they getting dirty from? Are you keeping them on newspaper or colored sand or anything like that?
What exactly makes them look like they need a wash? Could it just be their skin getting dull before a shed?
I'd say just follow the directions for a soak if you think they REALLY need a wash, but not too often, too much humidity can be a bad thing.
Just wait till RIGHT before they shed, when old skin is gray because it already separated from the new skin. The first time I saw that I freaked out for a minute and thought my gecko was dying.
If only one of your geckos smells the first thing I'd recommend is getting out the poo asap whenever you notice it, that's probably the source of the smell. There could also be something going in his digestive system making his poo extra smelly.
Mine get a bit smelly under the arms...I started using a little deodorant on them. Mitchum works best. They seem to like the unscented....the fragrances seem to make them sneeze. They prefer a little deodorant as opposed to a sponge bath.