Jan made up the schematics, so I'll let him address this.
The main consideration was to place them as close to the forward edge of the robot as possible so that you have the maximum amount of time to see the line and react. Putting them in a straight line doesn't really do anything for you except move them closer to the center of the robot and decrease the time you have to react. Having played with it, I really like having all of the sensors the same distance from the center of the robot.
There are three sensors close together along the center of the front edge that are intended for line following. Their positioning is such that a 3/4-inch line will not get lost in gaps between these three sensors; at least one of these three sensors will always be over such a line if you are reasonably centered, and three sensors give you enough of a span to be able to reliably stay on the line if your algorithm is tuned (as you can see from the demo line-following video).
The two outside sensors help if you're following a line with very sharp curves. Furthermore, they become useful when you're on a line course that has intersections or branches, such as a line maze. If you are following the line reasonably well, these outside sensors will be sufficiently far from the main line that they don't see it at all. Therefore, if they do see something, you know you have detected an intersection (or a very sharp curve).
We chose five sensors because we wanted at least three closely packed central ones for line-following to go along with the two outside sensors. We briefly considered going with six sensors (i.e. four central ones), but we decided it was more useful to leave an additional digital I/O for the user.
Does this sufficiently answer your questions?