Our observation is, then, that in the “real world,” the way in which most engineers default to working with EQ is to generally boost when shaping a tone that lacks a band of frequencies, but to cut when correcting problems.

When should you use EQ?

Only use EQ to remove nasty frequencies (by cutting) or to change the character of a sound and add interest (by boosting). You CAN'T use EQ to make a poor recording sound good. You can't add stuff that isn't there – only emphasize stuff that is.

What is boosting in EQ?

When you boost, you're manipulating the phase relationship of the frequencies, introducing a lot more gain as well as potential noise to your tracks. Use subtractive EQ instead, it's a much cleaner alternative. By subtracting the frequencies you don't want, you're subjectively boosting the frequencies that you want.