In my view, it's what the manufacturer says it should be--that's the person who designed the thing, he/she knows best.
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This is a no brainer.
It's your room and your ears. Keep solo vocals focused in the center. The speakers disappear. You'll find you can do this with either configuration. Then you have to decide if tweeters on the inside fit your taste for a more "intimate" soundstage. Most go for a wide and deep soundstage but your preferences in music genre are what matters most.
Speaker placement/room acoustics is at least 50% of this whole gig. IMHO
The physical distance between woofer and tweeter causes the sonic axis of the speaker system to be angled. (The ubiquitous MTM driver array avoids this). So, by positioning the speakers at an angle, the (small) effect of having the tweeters inside or outside can be counteracted.
However, the direction of the sonic axis deflection is such that having the tweeters outside is equivalent to positioning the speakers angled in. This means that you can probably leave them physically straight, and from a cosmetic point of view this may be desirable.