Generally, you shovel out a parking spot in order to get into your car and drive it away.  If you're in a rural or even a suburban neighborhood, there's little likelihood that your spot will be claimed by someone else.

If you're in an urban neighborhood, like New York City (where I grew up), I cannot imagine parking spots left free on the streets just because they were shoveled out by someone else.  It's not part of the New York culture.

What's left is the sort of situation you find in the so-called "outer boroughs" of New York - one- and two-story houses packed tightly on the streets, with narrow driveways and often no garage.  The residents of many of these house park one or two cars in the driveway and another in front of the house.  Parking's still at a premium in these neighborhoods, however, but it's here that people feel most proprietary about parking spots, especially those in front of their own homes.  I've seen many instances when homeowners would shovel their cars out from under the snow, pull out, and then stop the car and put trashcans filled with sand in the parking spot just vacated, to "reserve" it.

I think that any parking spots at the curb by houses of up to four families should be considered to be "owned" by that house when there's snow on the ground, and any obviously shovelled-out parking spot should be "reserved" for the shoveller until the snow's gone.  That means that the only spot a person would be entitled to claim would be in front of his or her own home - if you park in front of a neighbor's house, dig out in the morning, and then find your neighbor parked there when you get home, that's unfortunate - but it's your neighbor's house.

That last bit is tough, because in some areas the first snow comes along in November or even earlier, and there's some snow on the ground pretty much fulltime until March or April.  Still, considering the amount of work drivers involved, I think it's appropriate to recognize that work.

I was actually just reading about this with the blizzard around. I had no idea of the unwritten rules of the situation.

It turns out that you should in fact leave the spot open. The unwritten rule is - if you did the back breaking work then the spot is yours. There are even some cities that have this rule either on the books or as a sort of unwritten one that can be enforced. I'm very glad I don't live where I have to park on the street.

Another reason for leaving the spot is because there have been people killed over parking spots in the past. I would not want to risk my life because some crazy person would want my life in exchange for a parking space. There are also cases of vandalism for cars that take someone else's spot.

So while it sounds like an oxymoron since the space are on public spaces, snow fall does crazy things to people (maybe its the months of being cooped up in the house during the cold).