well...  bagginsbill might totally disagree with me, but i'd say you need both.  bagginsbill is a genius and an expert, but he's said in many places and times that he's not interested in the artistic aspect.  well, there's a reason most professional photographs are heavily postworked, even when they don't look as if they are.

just as a for instance, i was feeling really down about how not glossy my hair shader was in blonde and set out to look for source images.  i deliberately avoided press and magazine pics and tried for images more likely to be raw.  as a result, about the only shiny blondes i found were children.  and even they didn't have hair that shone white unless  the light hitting their skin blew it out.  i found some resources on doing glamor lighting that helped, but in all, the only pictures i found even with dark hair that had the kind of highlights people expect  were all shampoo ads.  which are about as likely to be untouched as photos of ice cream. 

so if i just looked at the math, i could adjust lots of aspects.  but in the end, i have to make stuff to suit the eyes, the same way a cook has to make stuff to suit the palate.  knowing  chemistry won't make you a good cook, but it will help you to make more complex combinations of foods work together.

eyeballing it is still important.  and sometimes you'll make huge changes in the numbers, and it will have a small effect on your picture. make the same changes with different lighting and camera angle, and boom, huge difference. 

the math is important, because you need to know what you're doing to do it again.  but don't lose sight of the effect you want, even if it isn't realistic.

at least in my experience, which has had, admittedly, mixed and slowly improving results.