Its a subject worthy of more than a simple two-choice poll.
As already stated, it really depends on the circumstances. If I'm in the process of overwhelming an opponent, to the extent that I'm taking multiple cities, I'll tend to only raze if its a large population; there's no wonders there; and its adjacent to strong cultural presence by my opponent.
I don't tend to experience too much problem with cities flipping back. Maybe if I've taken 4 or 5 cities over the course of a few turns, I may lose one of them. But provided I've already done the sensible thing of cutting of its supply routes, its likely I'll be able to take it back next turn anyway.
The other negative aspect of not trying to hold on to a city, is that it makes it much easier for one of you allies to step in and start claiming the territory. Personally, if I've fought hard to gain ground, I want to hang on to it!
That point always reminds me that one of the most useful units in a largescale land-expansion war, is the Settler. I always make sure I have a couple standing by ready to pull in and claim any newly available ground (especially if I have had to raze).