How do you frame night time, long exposure shots?

There are some great night-time shots on this forum.  From long exposure shots of the milky way with mountains off in the distance to city scapes to roads with car tailights/headlights streaming through.

I've recently begun getting into that a little myself.  My question is, when it's dark out, how do you frame these shots?  Some of the sky shots have extremely long exposures, so taking the shot and reframing based on the outcome can take a really, really long time.  Is that what people are doing or is there something I'm missing?