I always figured that they got clean by their very nature.

We squeeze/dump them out after each bath and keep them in a mesh net, so they seem to get fairly dry. I haven't noticed any mildew, at least. It may help that the bathroom window gets the full morning sun and it shines directly onto the tub wall where the net is.
I honestly don't think I would wash them unless I noticed an obvious mold/mildew/odor problem. Additional washing (in a tub of water/the dishwasher/the laundry) would be a waste of clean water, energy, soap and space to wash other things, imo.
I totally want a hole-less duck though.
