I didn't use the dash speaker mount locations. You are right, the dash and door are wired in parallel so there are actually 4 wires in the dash speaker harness - when you plug it in, you complete a parallel circuit down to the door speakers.
I was originally planning to try another set of speakers in the dash, but after listening to the components, and hearing how balanced they were, I decided not to bother - I have plenty of highs with just the one set and the staging is wonderful.
You could use components and put the tweeter up there if you want, but I find highs up on the windshield a bit harsh depending on the tweeter. Putting the mid and tweeter close to each other keeps them in phase since they are the same distance from your ear. I really can't tell that they are down so low - since we only have two ears, the sound still comes from in front of you. We can only identify a sound source on one plane, not in 3d space. Any clues your ears give you about source height is based only on the character of the sound and your brain taking the environment into account (echoes etc). You can certainly try both locations if you are on the fence. Let us know how it goes!