I think it all depends on the context. For example, if you are recording an album, professionally or not, at some point it's going to have to be prepared to go onto CD. And that's not just a case of setting the track order and burning. there's the issue of consistancy, both tonally and form a standpoint of volume. I'm not talking about loudening, I'm talking about making sure that track 2 isn't 10 times louder than track 1 etc etc. You don't want track 5 to have way more high end than track 8 if the musical style is the same. And then there's other stuff like fades, segues etc etc.
On a single song basis, I think it could be a different story.
Really, it all depends on what form you want the end product to be in, what you want to do with it, and what you hope to acheive with it.
I tend to put inverted commas around the word 'mastering' when I'm referring to what I do with my mixes. I recorded an album, and polished and prepped it for CD as described above. I didn't actually don't do much to the sound per se, beyond balancing the volume from track to track, and trying to acheive a consistant sound. Is that mastering? I dunno. Probably not. I know I do more than just fixing a shitty mix in mastering (well, I don't do that at all in mastering, I do it in mixing), but still, out of respect for the real mastering engineers here, I use the inverted commas.
I'm waffling. I'll stop now.