This is from my near fields manual. 
 

brconflict
...the tweeter and woofer need to be equidistant from your ears... 

Yes, and the most important thing is that the tweeter and the woofer are "in phase".
 
Some designs match the physical placement of the voice coils, other designs make adjustments in the crossovers, and many designs ignore the issue altogether.
 
I admire the designs where the mid point of each driver's voice coil is time aligned by physical placement. If this is the case the "front" of each speaker may or may not end up equidistant from the listener... but the mid point of the voice coils will be equidistant to the listener.
 
 
For example; This is an illustration of a time aligned 3 way system:

Hi all, and thanks for all the input.
With my original question pertaining to the vertical plane only, I wasn't expecting replies to expand as they have but, hey, it's all interesting reading anyway. :)

I use small JBL monitors (LSR2325) and have them mounted on the wall above and behind my LCDs, but aimed at the middle of where my head is when I'm mixing. Works fine...the tweeters are probably 18" above my head. With two 23" monitors, you don't have a lot of choice that doesn't put them too far apart.