hi
here is a mastering software- it is free and simple to use, if a bit limited-
http://www.cycleof5th.com/products/soundengine/?lang=en
if the link leaves you lost look here-
http://www.cycleof5th.com/download/?lang=en
scroll down the list until you see sound engine free 3.00 to download
as to your question - sure it makes sense to master, if the mastering improves the recording. anything you do to sound better is worth it. if your guitar or amp didnt sound good, youd do something about it if you could im sure, and the same should go for the music you create.
however im beginning to think that dwelling to much on things can be bad.
i too have little time, im working a lot. im pretty good with computers, and have no probs actually using them to record with, but i switched to recording on a daw some time ago, and have hundreds of gigabytes of crap to show for it. i can never finish anything, and am constantly frustrated in my efforts, as well as by the crazy amount of time im spending for little or no results.
i can however get awesome and proffesional sounding stuff on the fostex 4 track cassette machine i keep in my closet lol. if i record direct to it, then mix and master it through my mixing board and into my pc, i need do nothing else, and i get great levels in comparison to consumer cds, and its far easier and less time consuming for me.
so tread warily-