I agree, but with a twist. You always have pixels on the edge of an object that are half object and half background. If your sky color is the white preset and you are pasting onto a final scene that is black or dark you will see the edge in the composite picture. One thing you can do is render over the same sort of background color you will be pasting onto. Another thing you can do is to render your image double size or more from your final image size. When you scale the image the halo around your pasted transparency will be sampled into the object and the background and a lot of it will blend away.