Supposedly down the room not across it.
If you want the best stereo image each speaker should be the same distance from back walls and side walls (the distance from the side walls should be approx 1.5 times the distance from the back walls as a startkng point, possibly a little more).
Completely depends on the speakers and the room, in my experience.
Rear ported speakers tend to get a bit boomy against a wall and benefit from a bit of free space.
Front ported too, but not so critical.
Try it.
You might like it.
S’all personal taste at the end of the day.
I tend to ignore all that and just put them where I reckon they sound good and are convinient.
Currently firing across, not down.
Same distances from back and side walls though.
Opposite corners works well if you’re not arsed about imaging.