Having recently flown a lot in cloud, much of it below zero, over the last couple of months I can categorically state that icing is unpredictable, at least with the info available to us, so until you are there, or someone else has reported icing, then known icing (using the words as common language, rather than a legal definition) cannot exist.
I have encountered icing, not a problem since the aircraft is cleared for flight in known icing and the deicing systems functioned to keep the aircraft flying efficiently, but only on about half of the occasions I have been in cloud at below zero. In most of those cases the airframe icing did no more than slow me down by about 5 or 10 kts (I never allow prop icing to occur, but with boots I have to let the surface ice accrete). I have used the boots to recover performance, not because it was vital to flight safety. There has been one occasion when the icing would have been a hazard had I not used the deicing kit, and then if I'd have called an emergency London would have let me climb the 500 feet I needed to clear the cloud.
One thing I would say is if you are picking up icing don't use the autopilot! You need to know how the aircraft feels.
I would say then that anti-iced props are useful on their own. I would not like to fly with an assymetric prop, let alone two!