There isn't a "correct" answer to this. The general concept most people go for is called the "Wall of Sound". You want each track (instrument or vocal) to have its own place in the wall. It may be softer or louder dynamically than the other tracks around it and it may have its tone and/or sibilance at different parts of the EQ spectrum.
You generally try to mix each track into a slot allotted to it and mix EQ subtractively as much as possible. This may mean that for certain instruments, you take a little bit out from the EQ you would use if you were just using that track so that it makes room for other instruments, but it's generally more on a per track basis, not the whole mix.
In general, your best bet is to learn to listen for each track and figure out what tracks are interfering with each other. If You have a bass guitar and the low end of a piano that aren't getting along, decide which is more important and what frequencies are not overlapping well and cut the one that isn't fitting a little bit at the necessary frequency to give it some space.
I try to only deviate from where I'd normally want the EQ for a track by itself when it is necessary to make all the tracks fit together the way I want, so I do the minimum amount of adjusting necessary to fill out the wall of sound. Sometimes adjustments to the dynamics might also be a better solution, such as compressing a track if it is too loud sometimes but too quiet others.