Venkat, This is not strictly true. Depending on listener preference, there is a small range of positions behind the listener where you can place the surrounds. Dolby has a diagram here for 5.1 speaker layout:
Dolby - Dolby Speaker Setup Guide - Speaker Placement
If you look at the website for 5.1 placement, he talks about placing the RS and LS at 90 degrees to 110 degrees radius all to the
side of the viewer. What I have seen many people doing and many dealers recommending is the placement of the RS and LS BEHIND the viewer at more that 180 degrees, or directly perpendicular to the R and L speakers. This is wrong. These positions are occupied by the LB and RB speakers in a 7.1 set up, and not by the surround in a 5.1 set up. If you look at the diagrams on the manual of any AVR this is what they recommend.
If you are using a bipolar, it makes more sense to keep it directly in line with your ear as it has a wider dispersion angle.
The Dolby recommendations works for an ideal room where you can place the speakers where you want, and the room is acoustically treated. In real life, you have to struggle with parallel walls and untreated rooms.
In a set up I just installed for a friend, I had to place the Surround speakers at an acute 140 degree angle BEHIND the viewer for a 5.1 set up. He had already done the interiors and had drawn the cable routing. I spent two days trying various placement positions (the room was empty and I had free access), using cardboard boxes as speaker stands. I found that somewhere to the side of the viewer was the best position for surrounds.
But unfortunately, for aesthetics, I finally had to place the speakers behind the viewer. I had to adjust the speaker distance and the relative speaker amplitude to get the best surround effect, but even then the effects were wrong. For example, if there is a car driving from the left of the screen to the right, the sound starts from
behind you on the left and moves across the screen to
behind you on the right. It should actually start from your left and move to your right. The only time this is useful is when the actors are facing away from the screen and there is some action behind them. Then the sound should come from behind you. Most of the time, the actors are facing you so the sound effect is always to your sides and to your front, and sometimes above you for aerial effects. The advantage of bipolars is that they effectively handle the sound that is supposed to come from behind you, while a bookshelf will struggle to do that.
Of course if you install a 7.1 this is moire effective. But again there is very few sound that is coded for 7.1
I would recommend (particularly for Spiro) that you try a few positions and watch the same movie again to see which position provides the best sound for a 5.1 set up. You can use speaker stands if you have, or a tall cardboard box that you can move.
Another strange thing I have seen and that is something I do not understand. This has been emphasised more after seeing the Dolby site. When you use a stereo set up, the front speaker are always toed in to provide a stereophonic effect. But most instruction booklets and most dealers I have seen, keep the front speakers straight for an HT set up. I can understand this if you have a large seating area and 10 or more people watching a movie. But for small groups of two to four people, I think it makes more sense o toe in the front speakers. The Dolby site actually shows the front speaker toed in for a three seater position.
Cheers