It doesn't really seem to respect symmetrical areas on a model, unless I'm doing something wrong. If I have one symmetrical cluster spanning across the middle, it will sometimes come out disproportionately in the UV. Similarly, if I have two identical clusters on opposite sides, they will look different. I actually just had the right side of a jaw bone I'm making come out looking quite nice and the otherwise identical left side was essentially folded over itself.
I guess I could try doing something clever with deleting one side, mapping the other, mirroring it back, flipping clusters, etc but before I do I would like to know if there's some sort of function tucked away somewhere that i might have overlooked.