Depends on how tough it is to clean. I will always wipe down the windows and wind shields and head out, although I will let the defroster do the work on the side and rear window if they are icy. Modern defrosters are pretty effective, so why risk getting covered with snow or scratching the car? I don't go too crazy on the top of the car, especially if I am in business attire.
3 inches is my over/under to deal with snow on the roof. Less, and I feel like if someone is driving close enough behind me that snow coming off my car is a problem for them, they should stop tailgating. If it is more than 3 inches, I will brush the top layer off to get it down below 3 inches. I don't like running the brush along the roof because the brush can scratch.
I try to park in the sun or a garage if my car has ice and/or snow on it, even if it is a longer walk, because the sun will do a lot of the work for me. I don't park in the spot right next to the building if it is in the shade, because I risk re-freezing whatever melting happened up to that point.