I usually start by tightening all sides up all the way, and then loosen them all by the same amount, maybe 1 full turn or something. I just play with it to see if I like it, and if not I loosen or tighten as appropriate.
Finally I like to make sure its fairly equalized on opposite sides. I'll grab opposite sides of the cube, and turn them back and forth quickly. If the middle layer "sticks" to one side, then I will either loosen that side a bit more, or tighten the other side. I do this until both sides kinda move around without the middle tagging along.
Another thing that I sometimes do is grab the top corners of the cube and pull away from the center. You can tell if the sides are evenly matched this way. I don't think it's really important that they all match, but I think you might want to double check if there is a huge discrepancy.