You need both, flow and pressure. Low flow, and you can only run a single sprinkler or two. Low pressure, even with flow, and the sprinkler will not operate.
Some heads only need 15lbs pressure to run, like the Rainbird or Toro spray heads. A 15' radius spray head may suck up some water flow though... On the other hand, too much pressure is bad. A spray head made to run 15-30 psi will mist instead of spray if you run it at 45psi.
Rotors and impacts like both flow and pressure. Some want 60, 70, even 100psi. And, they want 15, 20, or more gpm.
There have been some good points in this thread. Head to head coverage is important, as is working with obstructions like trees.
One thing I see a lot that causes problems, is mixed heads on a circuit. Don't mix spray heads with rotors and impacts on the same circuit. They put out different volumes of water, and will cause fits trying to balance coverage. Either run all spray heads, all rotors, or all impacts on a single circuit.
As long as you're trenching, add some main lines too. I like to add hose bibs around the property.
Another thing I see is people tapping the hosebib on the back of the house for thier backyard water system, when the main connection is out front. What they don't realize is inside thier house, the water pipe drops to 1/2" which does not flow as much water. And, lots of time you can hear the water flow thru the house...
Go to a irrigation suppply house, like Greenmark or John Deere Landscape. They will have the Toro, Rainbird, Irritrol manuals(and other brands). The manuals have a lot of good info on them, about head selection, pipe size selection, friction loss ect.
