I used to raze most of the time, but I'll think I'll change if the new city is cut off from the enemy civ (no roads.) Here's why. Yesterday I played my first game at the 4th level...dammit, can't remember what it's called. Oh well.
The Zulus had built two cities that were in empty space in my third of the continent. You know the kind of cities...in little pockets on the coast I hadn't gotten control of yet. Well, BOTH CITIES flipped to me the turn or the turn after my workers built roads to them. So, here's what I think...if you cut off roads, you're more likely to avoid flipping. Lines of communications have a big impact.
Also, I tend not to be a real aggressive player. Most of the time, I snatch 2 or so cities, then sue for peace, consolidate. This means that the new cities aren't far from some of my cities with alot of culture. These cities I keep.
But I have razed when I start to get near the enemy core. Perhaps I'll keep them, after pillaging the roads connecting us to them.