I have done both, but I'm not sure it counts, because our gaming group has two standards for XP. One is the "you've slain the dragon, here's yer xp" variety, and the other is the "bring me a drink out of the kitchen and I'll give you xp" sort. With the first (and truly important) one, I can't recall anyone losing XP. With the second, people have gained as much as 50 xp for bringing said drink, or lost 10 xp for a hideously bad pun. Never enough to really matter in game terms, though.
As far as rewarding 0 goes, I have done so, but only for failure to show up for the game. The first couple of times, their character participated and received XP, but at the point when I determined that it was chronic (AND, they were repeatedly saying they'd show up and failing to do so without any real explanation), I "wrote" their character into a "parking place", if you take my meaning, and they stopped getting XP.