generally you should place surround speakers as symmetrically as possible. IMO you should try to keep the sub inside the circle the other speakers make and not in a corner. but most of all just play around with placement till it sounds right.
Actually, for maximum impact and efficiency, you want to corner load the sub (put it in a corner with relatively little confinement), as it will cause the room to act as a horn, and bass tends to be rather nondirectional. However, you will have to mess around with it a bit to get the best sound from your preferred listening location. I have a 2.1 setup with the sub in a corner, and it sounds great. As for the surrounds, for 5.1, you want them more off to the side than to the rear, and preferably as symmetrical as possible.
Here is the recommended THX layout for the best quality 5.1 sound:
Notice the sub is actually in the middle of the wall. This gives the clearest bass, but requires far more power to get the same level of bass in most cases. You can try it, but I have yet to find a computer system that is really sufficient for this type of loading, while putting the sub in the corner really helps it put out sound more efficiently (though some think it makes it slightly less clear). My best advice would be to mess around, and try several locations and go with what sounds best to you.