This question has no answer for me, because it really depends on my mood, inspiration, and what I'm actually making.
Sometimes I will draw out a basic plan
Sometimes I will just start building
And this is very rare, and really depends on the shape, I will go into creative mode and test it. If the shape is complex, or complex in Minecraft I will definitely do this to test out the best size. A good example of a time I've done this is when I decided to make a building with the shape of a hyperboloid (if you don't know what that is check the spoiler).