I always put it in the spot where there is most room left. Since in metal a lot of things are symmetric, I usually go with the opposite side of the active drum elements in the stereo spectrum. So if, for example, the part is mainly guided by a hihat on the left, I pan the harmony to the right. If there is a tom riff that bounces between the middle and the right (I pan drummer-perspective), I put the harmony/synthlead/whatever to the left. It balances things out and makes both elements easier to comprehend.