I would rather let it thaw as well. I don't see why there would be a difference in the bacteria by letting it thaw slowly. It also prevents having to use the extra cup of water and holding so much fluid.
One thing I have had trouble with is my donor having small BM. She is a 5' 2 100lb girl. When I don't have enough in the blender, I'm unable to get it to blend without using too much saline. Pretty frustrating.