There are two distinct activities that you need to consider when doing EQ:
- Taming frequency bands that have too much emphasis because the original signal path wasn’t “ideal.”
- Boost or attenuate frequency bands to allow each instrument to shine.
An example of #1 is using a super cardioid mic for a guitar amp instead of something more directed like a 609. As a result, you may pick up a lot of boominess from the surrounding space if you are recording in a non-treated room. So you would definitely want to use a HPF to kill anything below a certain frequency (I use 90Hz but that’s personal preference) to clean up any muddiness.
An example of #2 is adding sheen to vocal tracks. I use a Neumann TLM103 to record vocals, and it adds a certain brightness to the track. But even with this, I always add a high shelf EQ band around 2.5kHz (+3dB) to really allow the vocals to cut through the mix.
I realize the end result of both activities is that you’re tweaking the EQ, but I tend to approach the activity of mixing as two distinct actions: get the tracks to a good baseline sound and then go back and modify as needed.