One "professional" mastering engineer ripped me apart for suggesting that people leave some headroom when they mix, because "that doesn't matter any more with digital," and how stupid can I be.
Here's why it matters...
Even though the master out (outlined in yellow) is showing signal levels below zero, the True Peak reading (outlined in red) is showing about +4 dB over 0, which is the signal level after reconstruction through the D/A converter. This phenomenon is referred to as intersample distortion. Granted, the signal going on a CD or whatever will be below 0. However, whoever is mixing will be hearing 4 dB of peak clipping, which will give an inaccurate view of the mix (put a clipping plug-in on your master bus, hard-clip the top 4 dB, and then tell me you don't hear a difference).
This is why I recommend people leave 6 dB of headroom when mixing, and that's
what you give to the mastering engineer. If you want to follow the TP meter readings like a hawk to see if intersample distortion is occurring, by my guest...but it's a helluva easier just to mix with the peaks hitting -6 dB, and let the mastering engineer do the final tweaks. Well, a good mastering engineer, that is.