I had a scan the other day and the lady doing it said the best time was just after your period. I was ovulating at the time so it did muck things up a bit, but she was still able to see what she needed to.
I wonder if he said that because it is before you ovulate, and when the egg leaves the ovary it can look like a cyst - they call it a follicular cyst, so this way you would avoid having ovulation interfering with the test.