If you're flowering the general rule is every week you should drain and add new water and nutes. You don't have to wipe the res. I keep the slime on the walls all the way through the grow.
I have a mother room with a 4x8 tray and 70 gal res. It drinks so much water that the amount of new water and nutes I add keep it fresh. The plants look great...plus I'm not growing herb, just moms.
Generally speaking I feel that res cleanings are over emphasized and probably done too often.