Placing on snow is ok as long as there is little risk of a quick thaw and runoff - seed will wash away. I am personally waiting till the 6" of packed snow is gone in my side yard. I may likely miss my window but will still spread the seed - as long as it can get in contact with the soil it should be fine. That is the key - no matter what you do, if the seed doesn't make it to the soil it won't germinate.