Great comments! I would add that there is much debate regarding how "hot" one's mastered tracks should be relative to other people's music, vs preserving the dynamic range of one's own music (i.e. the "loudness war"). There is definitely a tradeoff, and you'll want to hear the difference. Luckily, most mastering engineers will for the same price give you a set of hot masters for the web / broadcast / iTunes (where dynamic range is less important than impact), and another full-range set for mastering to CD or vinyl.
IMHO, mastering is most useful in making tracks hang together as a collection. So, if your tracks a) all come from the same recording/mixing session, or b) aren't destined to be released as a CD or LP, one could argue that mastering just offers a final opportunity to tweak the eq, compression and loudness of your tracks. If that is indeed the case, and the untweaked versions sound just fine to you, heck, save your money.
In the end you have to trust your ears. When THEE DIRTYBEATS mastered its debut, we went with light compression (ie broader dynamic range) mastering for everything. That just seemed to sound more like the vintage garage rock records we loved.
THEE DIRTYBEATS - maximum vintage garage
Free download at http://theedirtybeats.bandcamp.com