I think alot of it is personal taste. When you listen to songs on the radio, different songs have predominately heavy bass lines, or drums. I always try to keep the instruments mixed to how it sounds good to me. Sometimes a MIDI bass is louder than a Real Track Bass, and likewise the drums or whatever. If I want vocals to be "up front" which is usually what you want, I run the instruments at 100-110, (max) which leaves "Headroom" (up to 127) for your vocals, so they don't get buried in the mix. To make sure vocals are heard, one could make sure , say, a lead guitar isn't walking all over the vocals at a particular part. Much is personal taste. When I first got into songwriting, I listened to advice and read about your writing style had to be AABA, or ABAB. Then I hear hot songs on the radio, climbing the charts in an entirely different arrangement. So there you go.