Let's separate tracking, mixing and the final product here though.
The -18dBFS theory for the AVERAGE of you tracks is a very sound principle. This equates to the AVERAGE being 0dBu on a VU meter and leaves lots of room for peaks to go well above that without hitting clipping.
If you start with your tracks averaging around -18dBFS, it helps your mixing because it gives you room to add tracks together. Every time you add an extra track the levels go up. If your tracked levels don't leave room for this, you just have to turn everything down while mixing anyway.
The final product, though, might as well have peaks up right to 0dBFS (I use -.1dB as my top level). Why? Because everybody expects it. Your iPod or car stereo with have the volume set for commercial tracks that are up at that level--and if your final, mastered mix is lower you'll just have to turn up the volume anyway, then get annoyed when the next commercial track booms out and deafens you.
The better debate is "how much dynamic range to leave in your final, mastered mix". For this there's no right answer. A lot of commercial stuff is compressed to within an inch of its life--if you look at the waveform in your DAW it'll just be a rectangular block with no peaks and troughs. This makes it seem loud...but also (to my ear) takes away the life in the music. However, you also have to consider various listening conditions when thinking about how much compression to use. A huge dynamic range sounds great in your home studio, heavily treated room. It also sounds good in you living room on your stereo if the house is quiet.
However, when you listen to you iPod on the bus or stereo in your car, the background noises will soon become problematic. You'll either lose all the quiet bits or have it turned up so much that the loud parts deafen you--perhaps literally if you're on earbud headphones.
This also becomes an issue when I'm doing music or effects for live theatre. Theatres are surprisingly noisy places with air con noise, rustling, breathing, coughing audience members and the modern scourge of moving lights. I have to keep my dynamic range much smaller (through the use of compression) than sounds right in the studio.
But I digress...your final, mastered mix might as well be right up at 0dBFS...but you creep up on this number during the tracking and mixing process...you don't start with every raw track there.
Bob
