In regards to the flashing that people have mentioned, I am very liberal in the use of sandpaper on my discs.
When I get in a new order of discs, and choose one to try out, the first thing I do is look it over and sand out any rough or sharp edges on the outer and bottom rims.
It is my opinion that most every disc can use a bit of work to improve it. After all, these things are being mass produced, and it is not likely that any will be perfect out of the box.
I find that drivers generally need less work to get into a comfortable feel and condition. My DX and KC Pro Rocs, on the other hand, always require extensive sanding all along the outer rim to get rid of the line that results from the molding process.
Most of the people that I talk to about sanding discs have never tried it themselves before. But, when they feel the ones that I have sanded, and then hold a new disc out of the box, they admit that the difference is significant.
I use typical sanding sponges with medium to fine grit for all my sanding. Try it yourself to see that it does improve the hand feel of the discs.