-34 degrees is a different story altogether. Propane has almost no vapor pressure at that temperature, so it requires not just keeping the fresh water lines warm, but keeping the propane warm enough to keep flowing. In those temperatures, pretty much nothing will do the job next to having some type of propane furnace or other heater and a way to circulate air between the bottom of the rig and the water pipe.
However, for temperatures that go from 32 to 0 degrees (0 to -18 degrees C), a heater strip will be of use.
It all depends on location. If I were in northern Texas or Oklahoma, a heat tape would definitely be of use. NoDak? Completely different story.