+1
You can clean the sand with every water change if you want (if you're on a two week schedule) but it isn't necessary. Some folks like to keep the sand a bit cleaner looking and that's fine (the sand under the rocks and the rocks themselves will have plenty of bacteria and other crawlies to repopulate it in no time). The major benefit of a sand bed (besides aesthetic reasons) is that it helps to level out the important elements of the water (alk, calc, mag etc) but the down side is that it can store huge amounts of the things that can cause problems (nutrients, phosphates, silicates) if left uncleaned over time. This is the argument that all the bare bottom tank folks make, but if you look at the amount of two part solutions or dosers they have to run to keep the tank in check it's higher than those who have a sand bed... Hope that helped.