If the song sounds fine after you've mixed it, you don't need to master it. I always aim for that.
But chances are, you'll find stuff like: you record 3 songs, and one is softer than the rest. So you go and boost that one song up.
Or maybe when you compare your songs to your favorite CD, they're all dull. So you go back and tweak your songs to make them brighter.
That's the kind of stuff mastering might consist of.
What I do is find the best recorded example of the type of music I can find, and put that on my DAW with my songs, and compare them.
The more you can just record things the way you want them in the end, and not have to tweak 'em to death with eq, the better. My best recorded stuff comes from doing that and not needing any eq. I really avoid eq if I can, but usually end up using some.