Personally, I just go by what sounds best in my ears. You already know what genre I normally do (epic filmscore shit for those who don't), so normally I just go by what instrument should be louder. But in order to make this apply for all genres, I'll just go by this:
What instrument do you want to be your main melody? Whatever instrument it is should be the loudest one. The others should be a few decibels lower in volume than that instrument. For drums, just edit the volume until you feel that they're good. Don't go by some uniform standard such as -6db because it won't matter if the rest of your song is -12db or +5db.
And normally when I run into the problem of the whole mix being too high in volume, I do turn down the overall volume, at least until I get to the mastering part.
Mastering part is essential for me, because it allows me to clear up any discrepancies with the overall mix and the maximum volume that I want to achieve (0db). Usually, what I do at this stage is load up Maximus and adjust the gain knobs so that the amount I want compressed is the only thing getting compressed, and watching the peaks on the mixer's analyzer. It's okay if the peaks go up to +1db because that's only transients getting clipped for only a few milliseconds, barely audible even on the best headphones (I know I don't hear it haha).
And now you know.....
And knowing is half the battle!