I like having at least two major territories for the fish with plently of places to hang out and not be seen by other fish. I like to have decor on the sides and leave the open area for a large open swimming area. Whenever I place things in the middle it seems to cause the tank to only have one territory. This may be because my fish are used to the setup I have, not sure.
A fishkeeper I met a few months ago kept telling me that fish aren't "aggressive", just more "territorial" than others. Some fish just need more space than others to feel comfortable. How territorial a fish will be depends on many factors, but if you follow general guidelines and advice, you can help prevent major problems. My experiences have shown that most of the advice and recommendations hold true over the long term. Most of your fish will get along when small, but once they start getting to 5 inches or so they really start to need more tank space to feel comfortable.