actually, each person's body temperature differs. the other may have a 37.5 degree temp and still feels ok. some have a 36.5 average body temperature.
im not a doctor, but the most logical thing to do is if the person's body temperature is around 38 to 38.5, its time to give something for the fever.
but please take note that the fever is not the illness by itself. it could be a symptom of something. example... some people, nurse a slight fever (sinat), if they have some sort of cold (sipon). some people, because of for let's say, being tired, cause their body temperatures to rise slightly.
the most general rule is that if the person feels bad, don't give medicines right away. the body's immune system might just do the trick.