Firstly, trust your ears always.
Secondly, I wish I had used a spectrum analyser from day one. I didn't truly understand eq relationships until I was able to compare solo'd tracks with an analyser. Then put the whole mix through the analyser and see where certain instruments had build up problems, or harshness. Also, our hearing changes from day to day, but the analyser is constant.
Try this: solo the bass and 1 guitar track. Pan the bass left, pan the guitar right. The analyzer will show the bass on top and the gtr on bottom. Scan the low mids on both and see if they both build up at a specific/same freq. Now do a surgical cut at that freq. on one or the other then listen in mono. They fit together like a glove now, and you didn't guess what the freq. was, you knew exactly what it was and solved the problem.
It's a great tool when used this way, but not necessary for broader eq cuts or boosts.
Another thing I like to do is see exactly what the strongest low freq of the snare is and then do a gentle boost an octave below that...i.e. snare hit is loudest at 228hz so I boost at 114. Supporting the fundemental in a precise(albeit overly-nerdy) way.
Helps in lots of other ways too...
Jay