Horizontal placement doesn't work very well, in my experience, unless the speakers are designed for that. If you place "vertical designed" speakers on their side, as you move your head horizontally you will be crossing through the separate lobes from the tweeter and woofer, and the perceived tone can change -- sometimes dramatically, depending on the speaker design. It's called comb filtering, and many speaker manufacturers caution against this placement. If you look at 2-way home theater center channel speakers that are designed for on-side orientation, you'll see that the tweeter is often shifted off-center from the woofer to minimize these effects.
You can of course easily try this yourself, it's usually easy to hear as you turn your head. I find I don't like to be so restricted in movement when mixing; I prefer to listen from several different positions. But if you don't mind staying in one place, it can work.
For 2-drive vertical designs, the null of minimum phase difference is usually midway between the tweeter and woofer. I try to set that height at ear level. If the monitor stands or desk are too tall to allow this, I far prefer to just turn the speaker upside down, with the tweeter at the bottom. As long as it's clear of the stand base or mixer panel, so you don't get too much reflection from those, this placement can work very well.
Here's a snip from Tannoy to show the sound wave interference:
Steve