When it comes to EQ'ing keys, there is a big difference between what you will do for electronic keys/synthesizers and acoustic pianos. So, let's go through both.
Electronic keys don’t often need a lot of EQ, but you can fine-tune the sound by adjusting a few fundamental frequency ranges.
Tip: Clean up the mudiness.
Keys and synths can get a bit muddy in the 400-600Hz range. Use a peaking filter with a Q value around 4 in this area to clean up the sound, especially when it is layered with other instruments.
Tip: Help the keys 'cut through' the mix.
If you need the sound to cut through the mix, try boosting slightly in the 1-2 kHz range. Or, you may even need to cut this area to make room for other instruments.
The 3-4kHz range is where the the primary presence and clarity can be found. Boost this area a little if necessary. Or, you can cut this area to make the tone a bit darker.
Regarding pianos, there are many different types and sizes that will have a range of tonal properties, so these tips will depend on size, playing style and miking techniques.
Tip: Cut the boominess.
Pianos can tend to be really boomy in the 100-200Hz range. The best way to fix this problem is with a low shelf filter at about 200Hz and cut 3-6dB.
This can also help reduce feedback or other low end resonance from drums or nearby instruments on stage.
Tip: Brighten the tone of the piano.
You can brighten the tone and help the piano cut through the mix by using a peaking filter in the 3kHz range with a Q value around 4 and applying a slight boost.
Keep in mind, too much emphasis in this range can exaggerate distracting elements like damper and string noise.
When you are dialing in EQ, don’t forget to experiment with the Q value. A lower Q value will give you a smoother frequency response and blended tone, whereas a higher Q value will give you greater control over the frequencies that you do and don’t want to cut.
It just takes a bit of time and close listening to find the sweet spot.