For near-fields,
IN GENERAL - with more exceptions than the U.S. tax code

...
High frequencies are more directional with poorer dispersion, so for mix accuracy you want to have the tweeters at ear level generally pointed in towards the ears, but you'll also want to play back with your ears off-axis to make sire they still sound good.
Low frequencies are less directional with much wider dispersion, so their level in regard to the ears is not as critical. However, they are more succeptable to both physical coupling to the desk (which is why it is ofteh *
generally* recommended to either place the monitors on physically isolating material or on isolated stands) and to near-field reflections and phase noding off of the desk surface.
For the later low freq reflection situation, you'd have to expiriment and see what works best for your environment. Before I got my 824s I was using some Klipsch two-ways on stands that worked best for me by actually standing the speakers upside down. That way the tweeters were more in line with my listening position and the woofs had better clearance and a higher incidence of angle to my old desk, making the bass cleaner and tighter. However, when I got my 824s and new high-density desk, I have them mounted vertically (normally) on the overbridge wings (with acoustic isolation between the wings and the overbridge surfaces), and they work and sound great. (Many will say that putting near-fields on a desk overbridge is a Bozo-No-No. That is the general theory, but in my acoustical sitution that is a bunch of hooey.)
I'd actually try the 824s sideways, as right now the tweets are just above my ear's level, but with two computer monitors and a video monitor in the center of the overbridge, I just don't have the room. Beside, It works great for me the way they are, so I am reticent to "fix" what doesn't sound "broke"

I bring that up to make a completly non-technical but pertenant point: Most nearfield speaker positioning vertical vs. horizontal is - rightly or wrongly - done as a matter of practicality more than a matter of anal acoustic perfection. More times than I care to admit I have been in installations where they had their monitors set vertical or horizontal for line of sight reasons or visual asthetics and not because of a major difference in sound. I don't for a millisecond advocate doing that, but it's a good thing to keep in mind next time you're paging through a copy of Mix and noticing that studio X has their nearfields laying on the X axis and studio Y has them on the Y axis. It's not always because they put a lot of acoustical thought into it.
G.