@lnxw48a1 Well, don't think of -18°C or +38°C as those limit temperatures then.
As you said, those are approximate numbers, so it's just circular logic to say that round Fahrenheit degrees are more round than the corresponding degrees Celsius. Just round degrees Celsius then, to -20°C and +40°C. Which do have symmetry as well.
But it's not like running naked in wind with -10°C is safe, so the margin of error is way too big either way.
As a counterargument I can say that 0°C being the point of water freezing (under "normal" conditions) is useful in daily lives too, as it affects weather, reliability of ice, all that kind of stuff.