As I said, for me they didn't seem to leave any snow if they were below Y=65 in a plains biome.
The Wiki says that the temperature has to be below 0.81 and the temperature _is_ dependant on the altitude.
I don't think it should matter, plains are supposed to have a temperature of 0.8 (which is below 0.81) and the Wiki says the temperature drops above Y=64 but doesn't change below Y=64, but I didn't get snow trails at Y=64 even though I did at 65-70.
According to the Wiki they only leave snow in biomes with a temperature of less than 0.81, though since Plains is 0.8 it should happen anywhere (temperature decreases with elevation above y=64, so Plains will be less than 0.8 at y=65 and above. The source code from 1.6.4 uses 0.8 as the threshold, which is likely the real value in current versions since the temperature only decreases above y=64, otherwise it is always at the base value). I don't think this mechanic has changed since 1.7, when temperature decreasing with altitude was implemented (they never leave snow in Plains in 1.6.4).