Whats the point of having individual volume and gain faders on a mixing console in addition to having a master volume?
Its just more ways of tweaking the sound. You don't want any of the volume faders to be too high, except for the channel faders on your mixer, they are fine going all the way up for scratching purposes.
I have a Vestax 07 Pro, and I usually leave the master volume fader on the preset line they have on the plate, about 75% up.
The gain knobs for each channel drift between 50% (the 12 o'clock position), and 75% (about the 3 o'clock position) depending on if I'm playing a single, LP, or feeding to the discman I use to play CD's.
It really doesn't matter where your levels are as long as you're not getting any distortion or clipping.
Peace!