You are correct Harris, you would use crease when adding edge loops creates not only unnecessary geometry, but when it makes crazy long cuts that you don’t need to be going all around your model. And correct again that since creases don’t make for the best results, it’s better to use them sparingly, in areas that are mostly hidden or just aren’t going to be noticed so much. But just because your loops is going to create an Ngon is not reason enough to go with creasing in my opinion, just fix your geometry to get rid of the Ngon. And the only rule of thumb I can think of for crease is just that since they make the edge kinda muddy, you have to use them in areas that are not going to be seen too much.
