The best sound engineers, indeed the best musician support personnel in any endeavor, will always at least politely listen to the concerns of musicians. They then use their best judgment to get the best sound and mix possible. Sometimes they agree with the suggestions they receive, sometimes they don't but they have the diplomatic skills to not openly refute your opinion as a musician. They get the last word by their actions, not their words, since they are the ones out there behind the console twiddling the knobs and faders, and you're stuck up on the stage, with arguably the worst sound perspective in the room pounding in your ears.
I always make a few brief, well-chosen, non-confrontational comments to the sound guy, tips and suggestions about various aspects of my rig and microphone, and how they've been best utilized in the past, and then just cross my fingers and hope for the best. Realistically, there's not much more you can do.
I've had extremely good relationships with virtually all the soundpeople I've worked with in the Denver/Metro area. It's a matter of building a rapport based on trust and respect.