If the tweeters are offset they should be on the inside edge and NOT the outside.
As far as I understand it, tweeters are offset to minimize cabinet edge diffraction at high frequencies. High frequency sound coming from a tweeter will interact with cabinet edges, causing small peaks and valleys in the frequency range of about 4 to 8 kHz. The exact frequency is determined by the distance from the tweeter to the cabinet edge.
If the tweeter is centered in a narrow cabinet, there will be two tweeter-to-edge distances that are the same. They will sum together, creating the possibility that the larger peaks and valleys will be more audible. If the tweeter is slightly off center, there will be two slightly different tweeter-to-edge distances. They may generate more peaks and valleys, but they will be smaller in amplitude. The idea is that these more numerous but smaller diffraction peaks are less likely to be heard.
A crossover is designed and optimized with the tweeter mounted either centered or off center. This is done by while measuring a single speaker. No stereo imaging is considered during that process.
People often ask whether speakers with offset tweeters should be placed with the tweeters toward the center or toward the outside. There is no clear answer, but it's easy to listen both ways. I tried this with my own speakers – I cannot tell any difference.
Most speakers that create poor stereo images suffer from some lack of response in the upper mid range. In a 2-way design, this is usually a problem that can be fixed in the crossover. I do agree with TLS Guy on that.
Interesting observation about offset tweeters. The Audioengine 5+ I use in my desktop system have offset tweeters and AE specifically recommends placing the speakers so that the tweeters are offset to the outside. I'm not sure it matters in a near field setup like mine, but interesting nonetheless.
That sounds to me more like marketing talk than speaker designer talk.
