On a regular basis you need to rinse all of your media. Cannisters are nice, because you don't have to do it quite as often. If your tank is fairly new, then I wouldn't, you want a lot of Benefical Bacteria growing in your filter, so when you do rinse it, you don't rinse it all away. It is also going to depend on the bio load in your tank as well. Last check you only had 2 fish, so I am not too worried about your bioload. But it you are cracking the case to rinse the sleeve, it only takes a minute to look at the media compartment and if it looks sludgy, then you should rinse it.
You observations are going to be the key whether or not you need to thoroughly clean your cannister. I usually clean mine at least every 3 - 4 months - but if your tank is crowded and/or you feed your fish alot, then you may have to do it more often.