I tend to never, ever, EVER let any level get hotter that -6dBFS in digital.

There is SOOOOO much headroom in 24-bit, that there's is no reason to get anywhere near the red.

If you keep the "meat" of the signal around -14dBfs and allow for occasional peaks beyond that, your essentially riding at 0dBVU - which is where pretty much everything else is designed to work at anyway.

Putting it another way, I get mixes in here that are too hot and need to be attenuated before moving on to "step 2" on a regular basis.

On the other hand, I don't believe I EVER received mixes that were "too quiet" to be worked on. Many studios that send me projects on a regular basis have adopted -12dBFS PEAKS as the virtual ceiling - And that's just fine.

It comes down to probably the biggest misunderstanding and misinformation in recording - High bit depth allows for more accurate signals at lower levels. That was the entire point of digital in the first place - even at 16 bit.

At 24-bit, where any signal that PEAKS above -47dBFS will have a higher usable resolution than a 16-bit signal, there is nothing to gain, and everything to lose by keeping the levels too hot.

HEADROOM is GOOD room.

but if I set the master fader so that the peaks only reach -3db to -6db when I open the file in wavelab it seems that the volume is even lower and that there are hardly any peaks at those values at all ! :cry:

but should I turn down the individual channels faders or should I turn down the master output fader ?

and what kind of scale is the cubase sx output bus meter ?

I'm not trying to be flippant or rude here, so apologies in advance if you take it this way. However, have you actually listened to your mix? Does it sound good?

If it does you've done your job, now let the mastering engineer get on with his/hers.

Your original question implies that you will be sending this to be mastered. Don't worry about hitting peaks near zero. Zero=bad.
Adjust your individual channels to control your mix volume. Don't move the main fader from zero when you print your final mix. The scale shows peak amplitude.