I always wash them because not doing so is too germy. However, I buy them at the last minute, wash them and set on baking racks in front of a pretty big fan, blot with paper towel and leave to air dry for several hours. Then I think the trick is not getting the strawberries too cool and not having the chocolate too hot. When the strawberries are refridgerated, it starts the breaking down process and if the chocolate is really hot, that cooks them and makes them juicy.
If you are trying to do them the night before, i suggest you just drizzle them in chocolate and not coat them. I put a piece of wax paper on a cookie sheet, dollop melted chocolate evenly on it, set the berries on the dollops, then drizzle well with dark, milk and white chocolate. Seems like the extra air space keeps them from getting gooey. hope that helps!