My gut says no to using snow for top offs. I know rain can contain contaminants from factories and such. I would think snow would too since it does start out as moisture though the levels may not be as high.
I would try to and get some purified water from your local store if you can. Now if you're snowed in and have no choice, then that's a different story. What I would do is melt the snow and then boil it. I'm not sure if boiling will kill anything since it started out as snow but why take a chance? Bring the temp down to the tank temp and then add.
But would I use snow over tap water? Yes as you won't have to deal with
chlorine. Or anything else that pipes can leach into the water.