It all depends on your room and your speakers, but all of the above is good advice. Try checking out one of the set-up formulas (like the one on the Cardas website). They take your room dimensions into account. The speaker to speaker distance, speaker to side wall distance, speaker to rear wall distance, and speaker to listener distance are all part of a formula based on your room dimensions. Try using the formula as a starting point and then make adjustments from there. My Merlins also recommend a slight toe-in, but that would depend on your speaker's design.
What determines speaker distance?
What determines the spacing between the speakers? I've seen a 1.3 formula for setting the distance between each. If I have a 9 foot distance from the speakers to the listening chair that would make the spacing approx 7 feet? Seems a little wide to get a good soundstage. Are there are any other "general" guidelines for getting a basic spacing between the two?