You need a decent monitor to be able to see it clearly, but there's a white halo around my masked objects when put on a transparent background. However, as you can see when I overlap one object over the other, that white halo is not really there. This was concerning when I first saw it -- after spending 10 minutes editing a photo, I realized that I messed up my mask, leaving white on the edge, and I might have to start over. Turns out that it was just a rendering error. Here's an example: