Hmmm. Not to sound confrontational but instruments have a fundamental frequency and a harmonic frequency. To know what those are is very important in making good clear mixes that are not fatiguing to the ears.
Here is some light reading:
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/sound/u11l4d.html
The main thing is to eliminate offending frequencies and enhance pleasing frequencies. Just saying and sparkle to a guitar or vocal is not how to approach mixing. There is masking that goes on and subtractive eq is the best practice in the opinion of most to achieve clear and well executed mixes.
There is no point in trying to eq instruments outside of there respective ranges as mostly that is detrimental.