I don't use filter sponges as I subscribe to the belief that I want all the poop to go into the skimmer to be extracted, not stay in a filter and break down into liquid (dissolved) ammonia and then nitrates. I think a filter reduces the effectiveness of the skimmer. But I have not done any scientific experementation to that. My tank water is crystal clear and my nitrates are 0 with no algae build-up in the DT.
I may suck some of the junk off the bottom of the sump but its mainly of a granular nature and of a coral dust or calcium consistancy. Its difficult to vaccum the sump as its on the floor almost, so it may only be a semi-annual event.