The rule that I am aware of is to use it when visible moisture is present AND the outside temperature is between -5 and +20ºC.
Now it seems that different rules circulate, such as -5º to +30ºC and -7ºC to +30º.
-15ºC seems a bit on the cold side I think.
So do not use carbu heat if you are above or below this range.
Using it above this range would not be a problem I guess as long as you make sure that you have no detonation and adjust mixture as required.
If you use it below the range however you could actually cause carburetor ice to be formed.
The reason that you should not use carbu heat below -5-ish temperature is that below this waterdroplets would normally be frozen and would just pass through the carburettor. If you would then use carbu heat, the droplets become liquid again and can then re-freeze in the carburetor.
As with most general rules, I guess that it is valid in most but not all cases b.