Chicago’s Rapid Transit Cycles reports that they replace more rear wheels to theft than they replace front wheels. Apparently, some cyclists believe the rear wheel is more difficult to remove than the front wheel, so they wrap a lock through the frame and front wheel but leave the rear wheel unlocked.
Reality, of course, is that the rear wheel is just as easy to remove as the front wheel. It can actually be a little faster, in fact, because rear wheel dropouts don’t typically have the “lawyer lips” to keep the wheel in place like front forks have. Adding to the pain is that the rear wheel is the most expensive part of the bike after the frame.
In low crime areas, I typically wrap my Kryptonite Mini lock through the rear wheel inside the rear triangle, just like Sheldon Brown illustrates in his article on lock strategy. Mary’s Bianchi Pista shown below is also locked like this. 
I also run a cable lock through the front wheel. If I need more security, I remove the front wheel and lock it against the frame and rear wheel using a larger U lock.