One very reasonable way is to use the ubiquitous golden ratio. If the distance from the center of the tweeter to the opposite sides of the cabinet (assuming rectangular face) are in the golden ratio
(8:5 :: outside:inside is a good approx.), then the baffle step response for both sides gets spaced out rather well. Rod Eliot (
http://www.sound.au.com) has a discussion of the baffle step response on his web page, and the Ariel/ME2 speakers (
www.aloha-audio.com) use the golden-ratio rule.
Another thing to think about is time-alignment. The sound emanates from the center of the driver. For a woofer, this is considerably behind the baffle, but for a tweeter, it is much closer, or sometimes ahead. Correcting for this is more important than it sounds -- a tweeter is basically responsible for harmonics and a midrange/woofer is basically responsible for fundamentals (also harmonics for bass, but that is an orthogonal issue). When the sound hits you, you don't want the harmonics to come before the fundamental. For both physical and psychoacoustic reasons, this is a bad thing.
There are a few strategies for this. A common one for TMs is to just tilt the front baffle. Sometimes you'll see these sorts of speakers in a pyramid shape; this also helps cancel internal resonances. The angle cuts you need to make it are harder than a rectangular cabinet, though.
If you have an MTM (or an MMMTMMM, in my case) you have no other option than to sink the tweeter behind the baffle. In this case, you often have a tradeoff between woofer-tweeter distance and the sinking depth. As long as the woofer-tweeter distance is within a wavelength of the crossover frequency, it'll be adequate. I think the best strategy is to try to achieve time alignment, but not at the cost of placing the woofer/tweeter further than this.
-Won