I would suggest, as I myself am practicing that, to NEVER master your own mixes, unless you're forced to (and even that, with a gun to forehead), because it's not that it would be bad, per se, it's just the fact that your music can get treated with another set (or more) of ears, which can spell a BIG difference in almost every aspect of "sonic picture"... Believe me, I've tried that and I absolutely stand behind that! Why? I'll try to give a brief explanation:
You are working on a song for about a week, give or take a day... You know every bit of it, every MISTAKE that was probably corrected/edited, basically you've heard the song so many times, it's in your head and you find yourself playing it (or a part of it) over and over again unconsciously... That can drive you nuts sometimes, and you can get used to the way it sounds, which means that you are ALREADY BIASED... By the time you've finished mixing you KNOW how it sounds, and you also EXPECT it to sound that way when mastered... And it does, which is the point here... Then you listen to it and compare to others... And believe me, when I say this, it ALWAYS sounds as if something's missing, maybe not at first, but after a while it does. I know, i tried it MANY times, even when i sent my mixes to mastering studio...
And it was such a relief and pleasure for ears, when i got back my mastered mixes, not that they sounded different, but kind of like, for example, imagine having a nice picture on your wall, and you don't swap it with other, just PUT THE PICTURE IN ANOTHER FRAME, and instantly, it seems refreshing, but NOT different! What I'm trying to say, it gets another point of view, which you as a mixing engineer CANNOT see...
And the same goes for another way around; when I get mixes for mastering, I just love doing it! It's a real refreshment and most importantly clients are satisfied...
But sometimes, when you'll get clients on a budget, be prepared to do mastering, if they want you to do it. A few weeks ago I did that for a band... They were happy, I was not... Can't help it!
Well, to sum it up, You're definitely missing out making your own masters. Period.
Cheers, Toni