I use 8mm collets in my DW621's all the time, particularly with the long dovetail cutters for the Leigh jig. They allow less flex in the cutter shank, therefore a cleaner cut and less chance of snapping.
I have a DW621K and only use the 8mm collet. I got mine from Toolshopdirect and, interestingly, whilst it has a DW part number it is described as a Trend Collet.