One of the handy aspects of getting boards assembled at Screaming Circuits is that we don't require fiducial marks for standard process boards. I would say that we build far more boards without fiducials than with. That's cool, but there are sometimes when fiducials really are a good idea. In fact, if you've got room on the PC board, they're always a good idea (just because something isn't required, doesn't mean that it's not a good idea).
Some boards are more in need of the marks than others. For example, not long ago, we got a rigid flex board in. It had three separate rigid boards connected by flex, designed to be folded into a stack. It looked pretty similar to the mock up in this image.
The boards didn't have any fiducial marks. Normally, what we do, is find a via hole, thru-hole pin hole, or some similar feature to use as a fiducial. That usually works, but not always. In this case, the length of the flex varied slightly from board to board. The PCB color was also very low contrast, which made it difficult for the machine to consistently recognize any mark we picked.
That meant our machines had a hard time finding the "home" spot, and we had to reset for each of the connected boards. Finding a spot on one board did not guarantee that we'd know where to place parts on the other two boards in the set.
In this case, it would have been far better if the boards were a consistent distance apart, and if each of the three boards had a set of fiducial marks.
What makes a good fidicual?
Most CAD packages have fiducial marks in their components library. Basically, it needs to be a metal dot surrounded by an area without any copper or solder mask. More than one is best. It should be an asymmetrical pattern that can only be oriented one way.
I've got some more details in this article here.
Routed up like a fiducial
Another rigid flex in the night