This is a subject of great debate. ML says 2 - 3 feet. I've heard it's best to observe the rule of thirds - that is one third the total legnth of the room from the back wall and one third the total width of the room from the side walls. Experiment and let your own tastes guide you.
Hola...well, I what I do if I were you, is place them where they sing most, and push them back when not listening...and the best distance your ears will tell you...trust in them! usually, 3 feet is the best...happy listening, and trust your ears,