Most manufacturers recommend you install the component system with tweeters closer to the midrange. A pillars and dash locations work best with certain component systems that are designed for it, or if you have the materials and know-how to make the location work out well for you such as building custom crossovers or time correction, etc. IMO, A pillars are best for tweeters that crossover at a higher point, around 5 khz and above. In the 3khz region (where most components crossover at) you have important vocals coming from both drivers, and if you seperate the drivers it will throw the imaging more out of whack.
In that case..what's the recommended distance between the mid-range and tweeter?