Tweeters should be as close to ear level as possible. Period. It's simple physics. The shorter the wavelength, the more directional it is. Unless it's an omnipolar design, the high frequencies from the tweeter as so directional that moving even a little bit will alter the sound. It is because of the tweeters that you can get such a dramatic difference in soundstaging, imaging, etc. by moving speakers even a few inches.
