Yeps... just make sure nothing clips on your channels or master and that everything sounds good in relation to each other. That's the main thing, really.
I've always learned that you want to give -3db of headroom for the mastering. -6db seems awfully low these days, but then again if they want it it softer.. they can turn the gain down and presto..
Personally I mix into my compressors and limiters and get my peaks up to -.01db. Now I wouldn't advise that if you're getting started and before you have a good feel for what each does to/for your sound,
but my idea behind this is that because it's all digital signal, I want as much resolution and scale for my dynamics as there is available.
Keeping them at -6db means everything else needs to be -6db softer as well. I work with 3d graphics too, so I'm kinda obsessed with maximizing resolution, lol.
Not because I want maximum loudness, but I want as much definition as I can. You shouldn't need the mastering to get everything nice and loud if you mix it right.
The other advantage is that, while you can't afford expensive masters, your stuff will still be relatively loud compared to the commercial releases... and won't fall flat in comparison, sounding all amateurish and boxy.
You also don't need to rely on what I call pseudo-mastering.. or the process of smacking some end-all mastering plug-in preset on your master bus, which will always mess with the sound, levels and dynamics you just spent hours crafting. Once you get to the stage where you have your stuff professionally mastered, they'll ask for your project files and stems anyway... the less time they have to spend on getting it loud, the more time they can spend on getting it beautiful.