I,m not sure where these numbers come from for preventing ice , so far we have had OAT of +5 to +10 banded about.
Most jet airliners will require the engine anti-ice to be swiched on at about +10C ( B737 NG) when visible moisture is presant but the Pitot heat is swiched on after start and off at the end of the flight what ever the weather.
I think that the numbers being banded about in the flying clubs have more to do with what some wannabe airline pilot instructors have picked up on the ATPL theory course than practical aplication.
The water in the pitot tube is not an issue as pitot heads have a small drain hole to clear this.
As far as I can see the two big factors for the use of pitot heat are accidental flight into icing conditions and the fact that the compass will have been swung with the pitot heat on.
Using the pitot heat has no penaltys except if you run it for to long on the ground , so as far as I can see if you are in any doubt as to if it should be used switch it on during the before take off checks and switch it off when you clean up after landing.