There is no right answer, other than perhaps "it depends".
There are, however, a number of really good starting points, and you've already heard several of them.
Every room is different, and more to the point, every pair of ears is different.
There are configurations I've seen that I expected to be horrible, and yet the engineer was able to create mixes that translated well to the world.
To be fair, there have been rooms where I just knew it was going to be awful, and it was<G>!
And to be equally fair, there have been rooms that looked right and sounded right, but I can't recall a case where things looked right and sounded bad. Does that mean there was never room for improvement? I'll never know!
So where do you start?
1) room symmetry, left to right - this is about as close to absolute as it gets.
2) distance to the back wall - try your best to get the back wall at least 11 feet from your ears. Can't always be done, but if it can it makes a big difference
3) loudspeaker placement (part 1) - horizontal or vertical, and free space, half space, or dog-house - every loudspeaker is designed to work optimally in one or more configurations, so you need to read the manual. If you need to rotate the tweeter to lie them on their sides (or stand them up) do so. Similarly, if there is a switch that tries to equalize the low end depending on placement use it. And don't be afraid to try alternate positions, they probably won't work, but if they do... Similarly again, don't be afraid to try monitors on their side even if the manual says otherwise. Worst case you'll learn why the manufacturer advised against it. About the only rule I've run across that seems somewhat universal is that it is a really bad idea to doghouse near field monitors, and it is a really bad idea to place monitors with rear-firing ports against a wall. I'm sure there are others<G>.
4) loudspeaker placement (part 2) - distance to ears, distance between - start with the oft-quoted equilateral triangle and a distance that looks right. Distance is a crap shoot! Experiment. It is the only way to know that you've found the sweet spot. Start by adjusting distance, and keeping the equilateral triangle. Once you've found a distance that sounds good play with separation between the loudspeakers. For distance you are primarily listening for tonal balance, although it will impact the stereo image a little. For separation you are primarily listening to the stereo image, but tonal balance can change too.
Ain't this fun?
For folks building a new room I recommend taking care of loudspeaker placement before moving any equipment into the room - it is usually easier to adjust equipment locations. Not always (a 12 foot wide console is going to take up a lot of space<G>), but usually.
And while listening/experimenting with loudspeaker placement use a LOT of different material, some of your own, and a bunch of your favorite albums. And if you have any well recorded 'naked' tracks listen to them too.
There is science here, but there is also art. Unless you are designing the monitors and the room from scratch the art part has a lot of impact. By way of example, my new Presonus S6 monitors are a lot closer than I expected. Thus far (and I'm far from finished) I haven't been able to screw up the stereo image much, but I have found the tonal balance to change more than expected as they move back and forth.
Another example - I have a pair of Urei 809s, and right now they are on stands in the room. They used to be mounted in the wall in my old place. There is no question I like them better that way, but I'm a ways off from building the next room, so here they stand (pun unavoidable?)