Ah now I understand
...
To start, one of the more difficult steps in mixing is knowing what to mix/change, and knowing what not to mix/change.
With that said, the answer is no... You do not only focus on scoping out the mid range of "mono or high end" instruments.
First, mono is not considered equalization whatsoever. Mono or stereo have to do with the stereo field, not an actual eq range. Therefore, both mono and stereo signals may need equalization within your session.
Scoping/cutting any frequency range in a sound is usually considered a better first step in equalization VS starting out boosting/increasing a frequency range in a sound. Nonetheless, any sound from a piano to a synth lead can possibly require an eq adjustment... It's all about what the actual sound in question needing... and that comes down to listening to the sound itself.
There is NO magical preset to equalize ANY particular instrument. With that said, some snares are going to need low, mid, or high equalization in your track, some will not. Some keyboards/synths are going to need low, mid, or high equalization in your track, some will not. It's all about listening to the sound in question.
Definitely look into the difference between a mono and stereo signal...
I hope this helped a bit