i usually run an empty load before i throw them in. also dont let the wifey see how dirty they are before you put them in the washer. they can get pretty nasty lol.
I have only cleaned the pond filter sponges once in a year on my 100gal sump and they were pretty clean, it think it is due to going through masses of k1 first and the rubbish settles at the bottom, I have siphoned it out and it nearly filled a bucket with gunk, this is 1 yr in. My external filter gets cleaned every few months and this does most of the fine work but I also run a sock every now and then.