I mix half a liter of methanol in the tail tank of a Ventus C. I don’t put any in the wing tanks which have a higher mass to surface area ratio and thus a longer thermal time constant. I’ve been doing this for 20 years and have not had a problem yet. Your mileage may vary.
This system was put to the test recently on a particularly cold (for the southwest U.S.) early-season flight at altitudes up to 17,500’. The OAT got down to -10 C and was probably below freezing for 2 hours straight.
I have often had water leak from the wing valves, run back into the gap between the flap and wing fairing, freeze, and jam the flaps (which are interconnected to the ailerons). As a previous poster noted, this is quite unpleasant. I was encouraged by the fact that this did not occur on my recent flight for which I had been particularly diligent with the application of Chap Stick to the mating surfaces of the valves prior to filling.
Other than the leakage, I have never dumped untreated water when the OAT is below freezing. Considering the flap jamming issue and the potential for freezing on tail surfaces, this would seem to be a bad idea.
Methanol (methyl alcohol) is readily available at hot rod shops, reasonably priced, and relatively benign environmentally (it will evaporate before reaching the ground).
Mike Koerner