I always rinse out as soon as the wort is out of it, usually with my cold hose that feeds my chiller, then later I'll go back and wash out with a soapy sponge. I'll use a scrubby if there's anything really caked on there. Rinse well, then let dry and put away. I try to store my kettles upside down, but I'll end up rinsing them out again when I start the next brew day.