It's better to come as close to the L/R speakers as possible. Speakers from different manufactures often sound quite a bit different. And in this hobby, even though these differences can be subtle, you don't always know what you're missing until you get it right.
Never. You want 3 identical speakers across the front. More and more movies are sending dialog to the L/R speakers if the actors are at the edges of the scene, not to mention effects that pan across the front. You will blow the illusion of a wide-soundstage if the sounds change as they move from speaker to speaker.
I have one more question. Last time I said the B word (Bose)I took quite a beating. I got a good deal a set of acoustimass 10 speakers and I was thinking I needed a center driving speaker. I haven't had a chance to listen to it the way it is because the room isn't quite done. I have a chance to get a bose vcs-10 for dirt cheap. So should I use the cube, pick up the center or just scratch the itch and listen to it when the room is done? (the new guy)
All I can say is audition numerous brands etc before you decide. I was never a fan of Bose myself but as long as you like them & how they sound thats all that matters....Happy listening.