The audio trim settings are there to adjust the input levels to match various input sources. Typically, a microphone will be in the -50 range and line level (from a mixer set to line output) will be much hotter.
You adjust the level in the camera to get the best signal to noise ratio (S/N). That way you avoid unwanted distortion (input too hot for the setting) or hiss (input too low for camera setting). If your camera audio input knobs are set way over 5 or under 2 or so, the input trim may need to be adjusted. Listen with good headphones for distortion or excess hiss.
For video: I try not to shoot over +6. It will get very grainy and look horrible when encoded to DVD. I read that the -3 setting cannot record 100 IRE, but I have never checked that.
Bob