I would caution that this depends on how the mixer is built. If the faders drive a VCA, you are correct. If the faders are pure analog variable resistors, it may actually be a cleaner signal with them all the way up and lower preamp gain.
The reason for this is that the mixer is actually boosting beyond unity and the fader is just cutting it back down (or occasionally vice-versa). You're going to have that gain up prior to the fader anyway, so you might as well not add an additional cut with the fader and additional gain at the preamp, as this will just add additional noise unnecessarily.
That is, of course, unless there's enough noise induced in the signal path between the preamp's output and the pre-fader gain stage, in which case it again becomes beneficial to have the fader at unity gain (or even lower if there's enough headroom) and have the preamp gain hotter, as the noise introduced in that short section of inter-stage wiring would be smaller proportional to the signal at that stage. This is, of course, unlikely.
The point is that there are no hard and fast rules. Without seeing a schematic for the mixer in question, these are only general suggestions. You have to experiment to see if the suggestions are correct for your particular mixer.
