I want to take a moment to talk a bit about signal polarity and its importance to your mixing.
Firstly, what do I mean by "polarity?" The polarity of a signal (also commonly called 'phase', though in truth the two are different -- whichever word you prefer is not critical for this discussion) can be described rather practically as whether or not the signal is perceived as moving toward you, the listener, or moving away from you. Sometimes this effect is quite easy to hear. For example, try miking an open back guitar cabinet from both the front and rear, simultaneously, without flipping the polarity on either mic. As the cabinet's speaker moves toward one mic, it moves away from the other. Now pan those signals hard left and right, and stand in the middle with your eyes closed, listening.
Feels like your head is being twisted off your neck, no? The sound feels like it's coming at you on one side, while on the other it feels like it's going away from you. They may both be the same level, but one will feel more present (the one coming toward you) while the other will feel more distant, or even quieter.
This is powerful stuff -- you can exploit this phenomenon to affect the perceived balance of elements within your overall mix!
With stereo signals out of polarity it is quite obvious to know which is moving in which direction, but what about mono signals? Mono tracks don't reveal their absolute polarity quite so obviously. For this reason I believe too many engineers dismiss the value of a signal's absolute polarity when mixing, since it may not be clashing obviously with another track, but its polarity can still be critically important to achieving the right balance in the mix.
As I mentioned briefly above, when sound is perceived as coming toward a listener it tends to have more authority -- this is true even of mono tracks. Put another way, sounds approaching the listener often feel louder, even though amplitude and polarity are not related. As a result, a signal's polarity can be manipulated as needed to alter the perception of its balance in a mix.
Put another way, signals 'coming at you' have more authority than those that feel as if they're moving away from you.
So how does this play out in practice? Well, if you find yourself raising the level of a signal trying to give it more importance in the mix, yet you find that at one point it is too loud, while any lower it feels too soft, try inverting its polarity to see what effect doing so may have on its presentation. It can also be helpful to invert the polarity of other tracks that may be competing with it, particularly those that may share similar frequency content. [There is also a good chance that you may need to add or alter some dynamics processing to balance things, but that is another post for another time].
One thing to realize is that the perception of absolute polarity is a visceral thing. You feel it, you don't really hear it (close your eyes as you listen, and try to feel the position of the track in the mix). It is also useful to note that the more LF content in the signal, the more noticeable its absolute polarity tends to be (that is, it's easier to perceive its polarity). Higher frequency signals aren't as obvious, though their polarity can still be critical when blending them with others.
Absolute polarity isn't a sexy topic -- certainly not one that garners much discussion on audio forums and in magazine articles, but it is a powerful tool for mixing. Oftentimes the most subtle tricks can have a profound impact on the final result. Respecting absolute polarity, IMO, definitely qualifies as one such technique.