Good question with no solid answer.
My recommendation: Only compress the master bus if the MIX actually NEEDS it. And then, only 2-3dB of gain reduction should do. If not, try revisiting the mix and find out where it's lacking. Save compression for the sake of volume to mastering. Even if you're only doing it yourself. The "rule of thumb" is to do nothing to the mix for the sake of sheer volume. If it needs something to "hold it together" a bit, do it. If it's just for volume, turn your monitors up.
The only type of file that will work on a standard audio CD is a 16-bit, 44.1kHz PCM file (.wav, .aiff, etc.). Again, through the mastering phase, you should be at a higher wordlength at the very least.
If you're in a DAW, you *probably* shouldn't go to that level until the very last minute. If you can work in 24-bit or 32-bit float, that's the place to be. If you're sending out for mastering, 24-bit 44.1kHz files are almost ideal.