I'd say night chasing is fine as long as you know what's going on, and having current radar imagery helps immensely in this manner. I've had some successful post-sunset tornado intercepts, including the tornado north of Anthony, KS, on 5-12-04 (the pics of which Dan posted above). June 10th 2004 comes to mind immediately as well as a good nighttime chase (tornado near Red Cloud NE).
That said, I tend to be much more cautious about night chasing (for obvious reasons). There's usually a time when I say "screw it" and call the chase off, more often than not because I no longer feel comfortable / safe in the position. When I was blasted by the RFD on the 5-9-03 OKC supercell, I opted to take the safe route south and end the chase. Similarly, on the 4-21-05 chase (from se KS into sw MO), Gabe and I reached a point after which we no longer felt safe chasing the storm. In fact, we got chased BY the storm for a bit, all while it was tornado-warned and had very intense inflow. During this particular chase, we were driving south on a country road and had generally weak northerly winds, while the wx radio was harping on the tornadic portion of the supercell to our immediate southwest. We reach an intersection with another county road, at which time leaves and small twigs began to fall from the sky. As we turned east, we were blasted by very strong northeastly winds, and we could see a nice wallcloud to our immediate west. After this time, we were just doing whatever we could to get out of the way of this storm, and we eventually did. Just make sure you always leave yourself a safe route out of the storm's path, since things can creep up on you very quickly at night.