As they occur just after sunset and the sky they are in is still pretty bright, you don't need a long exposure. In fact, I would slightly under-expose meaning your shutter would be on the faster side. It's easy to photograph sunsets with the camera hand held after the sun has gone down but you might need a tripod as it gets darker although resting your camera body on anything stable and still will suffice. I photographed the last main nacreous cloud we had here back in february 1996. It was a spectacular one too. I only had my 35mm camera at the time. I'd love to see another and photograph it with my current camera.
It was a very windy evening that day I remember.
Here's a rubbish scan of one of the pics I got. you can tell why I'd like to get more:P