I don't think you have to flush it every time. I would do it maybe once a month though. It can help flush out dirt, sand, etc... that can get sucked into the open loop cooling system.
:agree: I would only flush every once in a while, esp since you are in fresh water.
My manual says you should flush after every use in either salt or foul water, it says nothing about fresh...so just do it once a month like gerapau says!
