Good points there, makinao, but if I read the TS right, he wants to know specific frequencies he should adjust for a given mix.
I guess the short answer to that is... there isn't any. It all depends on how you want a mix to sound and what kind of music you're mixing. I think it's kinda weird to apply the same eq settings that you used on a death metal mix last night to the orchestral recording you're mixing today. While it's true that you can boost as well as cut, I believe the best way to explain this is: cut first, boost later. Granted that most new engineers more readily hear a boost rather than a cut, I think we should train our ears more 'to hear less'.
There is a technical explanation to cutting frequencies first. For one thing, boosting immediately often means gain on the outputs which can lead to distortion while not achieving the desired mix. In an analog mixer, we can get away with boosting (a lot) since hardware usually have sufficient headroom to accommodate the boost. With digital mixing, however, we hit a brick wall at 0 dBFS and we all know what happens when we 'try' to go above that... digital distortion (which is a whole less pleasing than the distortion we experience from analog gear).
This sort of answers why we use the mantra of 'cut first, boost later' since most new engineers are now mixing in a DAW and have no idea what it is to mix in the analog realm except when they get their hands on an analog mixer. Mixing in a computer, while technically more accurate (and this is to be a bone of contention with some people), is also unforgiving of digital overs which result in unflattering distortion. The recommended way to mix digitally is usually to cut first then boost later expressly for the purpose of avoiding flatlining at 0 dB. We can always cut offending frequencies first, then boost what we need in the latter or final stages of mixing or apply gain after eq.
We have to understand, however, that in order to hear the effects of proper eq, we absolutely need a trusted pair of monitors, a good room, and we need to have the gain staging of our equipment in proper order. What use is there in eq'ing if one part of the chain has low levels forcing you to boost frequencies needlessly or your room has a standing wave at your particular mix position? This is the reason why some amateur mixes either sound boomy or screechy.
