<p>The thing about the mixing engine in most modern DAWs is that it uses a kind of DSP calculation method (floating-point maths) that effectively gives you ludicrous amounts of headroom. As a result of this, you can be lighting up 0dBFS warning lights across every channel of your mixer, but you won't actually be clipping anything -- as long as you pull the master fader down so that you don't clip your master channel, and therefore your audio interface's output D-A converters (and indeed any bounced-out audio file of your mix). So the truth of the matter is that I don't typically give the master output level a tremendous amount of thought these days.</p>

<p>However, there are two provisos to this:</p>
<ul>
<li>Some plug-ins (particularly anything that's modelling analogue circuitry) may sound different depending on how hard you drive it, so you need to bear that in mind when processing. In practice, all this really means is that I sometimes use gain plug-ins in my plug-in chain to determine how hard certain processors are driven if I feel it's necessary.</li>
<li>I generally try to avoid going above 0dBFS on any individual channels if I can avoid it. This isn't because I'm worried about something clipping before it reaches the main mix buss (as I already mentioned), but rather because if I find I later have to bounce out stems or individual channels for any reason, then I can do so without there being any clipping bounced into those files. In practice, this means that I usually end up with my master buss peaking at least 6dB below 0dBFS, and often even more.</li>
</ul>

<p>As you can see, I'm far from systematic about it (in the way you had to be in the analogue domain,  or indeed back in the early days of fixed-point digital mixers), because I don't think it really makes any difference to the sound. Hope that helps! Incidentally, if you'd like a quick refresher on the the mechanics of how to manage digital audio gain structure in general, check out this great Hugh Robjohns article from <i>Sound On Sound</i>, which is now free to view: '<a href="http://www.soundonsound.com/techniques/digital-problems-practical-solutions">Digital Problems, Practical Solutions</a>'. </p>