If you use the "drape from zero pose" option in the simulation settings, you don't need to start your animation from the T-pose. However many users prefer the additional control that comes from draping that way. If you need to start your animation from zero, just render out the frames you want to use. You do render to individual image files, don't you? ;) Many users do this. It enables you to restart if Poser crashes half way through, or if you run out of time and have to cancel the rendering. Also, you get more control (there's that word again) over the way your movie is assembled and encoded. When you save the scene, the dynamics information is saved with it (in a file with a .dyn extension). So there's no need to re-run the simulation unless you change the pose.