I am curious if it is common place for a radiologist to want to start dictation of a study before it is final/completed (all images and measurements present)?

In general one would not make a final dictation until all studies are complete occasionally you can see quick interpretations based on emergent scenarios

Need more specifics. If the images were acquired but some post-processing for measurements or 3-D image generation had to be done, it would not be unusual to dictate based on what was available and finish when the processed images were done.