Remove It Regularly
I have to second Sabreman's comments. We need the knotmeter because it provides the info for the anonometer to give us true wind speed, but even if we didn't need it for that, I find knowing boat speed over water when compared to speed over ground is nice to know so that you know what the current is doing. Also, as someone mentioned the feedback from the knotmeter is much more instantanious than the GPS if you are trying to gage the effects of sail trim on your boat speed.
I remove my impeller everytime I leave the boat and re-insert it everytime I go sailing (assuming I don't forget). It's the only way to keep it clean and functional over any period of time. Fortunately, I have rather easy access to my impeller.
It took me a whole season before I had the nerve to remove the impeller when the boat was in the water. Now, I just do it routinely and wonder why I was so scared to do it before.