As for crossover slopes between a midwoofer (or midrange) and tweeter, in my experience, it's a balance. I think first and foremost, summing should be good. Directionality should be matched. Both of these are somewhat dependent on crossover frequency and slope.

If the loudspeaker is physically large, then each subsystem is probably further apart than a smaller speaker. For example, the midrange driver of a midhorn is probably further from the tweeter in a large horn speaker than the midwoofer is from the tweeter in a small mini-monitor with a 6.5" woofer and a 1" dome tweeter. To me, it stands to reason that localization of sounds from the mini-monitor would be difficult even just a few inches away. The horn might need a few feet to integrate properly. Then again, the horn probably sounds just fine 30 feet away, whereas the mini-monitor might be straining.

Some would offer the coaxial arrangement as a solution that allows large horn sound with the close-up integration of the mini-monitor. It definitely removes the problem of having a vertical stack, but I've never found one that I could live with the compromises it brings. Not to say there aren't some good coax and multi-driver horn/manifold setups out there. There are some really nice ones. They just aren't my cup of tea.