I also vary the background. If its a brighter coin like a nice silver ASE, halves, etc I use a black background. For anything toned or darker (copper) I use a white background. Although I've never tried a grey background for coins even though I have one. But as mentioned above, be careful of Alien circles.
Thanks for the recommendation. I've been playing with it tonight and it seems like a nice piece of software.
If you're working on an IOS device, the app Krop Circles allows for circular cropping, as the name implies (lol).