I was born and raised in a residential area of Brooklyn (Mill Basin), and there the chair situation depends on the amount of snow and the post-storm weather. If we got a ton of snow and it's sticking around for a while, we put our trash cans in our spots. For the storm a week ago we didn't do it because everything melted pretty quickly: that being said, my brother was fired up when he found our neighbors had ganked his shovel spot and he had to park around the corner next to a snow drift.
It's not an anti-outsider move in that neck of the woods, because no one goes to Mill Basin unless they live there. I have noticed if the part of Boston I live in (Brighton, on the border of Allston) uses chairs, but given the general awfulness of parking in my area, I could imagine that blood would be shed if a shoveled spot got jacked.
I'll close with an awful spot-shoveling story: a few years back my girlfriend at the time lived in the Harvard Square area, and she had a car with CT plates that was illegally parked during a really bad snow storm (it was parked in a residential spot, not in front of a driveway or anything). We arrived at the car to find a few tickets on the windshield, and we started chipping at the ice and digging the mofo out. It was a bit of a struggle, and I am a fat kid, so we took a break at the halfway point and went in to warm up for a bit. When we got back the car had been towed: the tow driver had waited until we had dug it out enough for him to snatch it, and off he went. I did not win many Tommy Points that day.