In a rock band, after compressing the various components in a mix you might have about 10db maybe more of dynamic range. I'd give it about -6 to -3 db head room between that and 0. 

Personally I like my punk rock mastered to death with maybe 3db dynamic range on the final mix tops. I also like it loud so I whack it right up to 0db when I think in general -4 is normal. (If it's for a DVD or the telly -10 is the legal limit).

Volume does not really come into it, (except that technically a very very quiet signal has less resolution), dynamic range is probably the only issue. If you brick wall or overload your mix there isn't much you can do about that in the mastering. 

If you are using tape, you can push the tape and saturate it for effect, if I was using tape, I would probably do this when first laying tracks. 

Interesting question, and I must confess I couldn't follow Atmoic Suplex's advice cause of sheer lack of knowledge. What is the "correct head room"?

As a homerecorder and a dilettante pretty untouched by recording theory I'd say "mix loud, master less", so, digitally speaking, the max of "original data" is there. If the original mix is less, things may get lost, I'd say. But this is just my opinion coming from working with silly shareware programs and homerecordings.

MIx with the correct head room, master correctly.

'Loudness' means almost nothing, especially in the digital world.