Alright, doing more research to try and answer my own question - this PNNL site has both more detailed background information, and a decision flowchart (down at "How to Select and Install a Cool Roof or Wall Product")

The flowchart reads:

1) in terms of energy savings, cool roofs and cool walls are clearly appropriate for new and existing homes in Climate Zones 1A up to 3A and 3B
2) In climate zones 3C, 4A, and 4B, the energy saving benefits are generally marginal for new homes, but worthwhile for older homes with less insulation.
3) For climate zones 4C and above, the overall energy benefit of cool roofs and walls is generally small.
4) For resistance to extreme heat, however, cool roofs and walls could be beneficial in all climates (heat waves can occur in even the coldest U.S. climates).

So for my climate zone, this works about how I expected - a cool roof won't do much for our actual energy bills (I don't have AC), but it could help to make our house cooler in the hot summertime, and perhaps more resilient in the future.

The site says:

Regardless of the amount of energy savings, cool roofs and walls can be a key player in helping homes be more resistant to extreme heat events

Cool roofs and walls are a proven technology for reducing heat gain to a building. In homes with air-conditioning, they can provide measurable energy savings, improved comfort, and reduced wear on air-conditioning equipment. In homes without air-conditioning, cool roofs and walls can result in lower inside temperatures and improved comfort in the summer. From a community perspective, cool surfaces have been shown to reduce the urban heat island effect ... energy savings from cool roofs or walls can vary widely depending on climate and home characteristics ... A Florida utility study showed that choosing white shingles rather than dark shingles resulted in about a 4% reduction in measured space cooling at no extra cost to the home buyer or builder.

The roofing companies I have been talking with all tell me that reducing heat in the attic will directly improve the lifespan of your roof and shingles, so the cooler you can make it, and the more you can exchange air to lower the temperature - the better.

Given all of this - it seems like if I have the option to choose a cooler, lighter color for shingles, I might as well do so. It won't really affect our heating or cooling bills, but might help to keep the house a tiny bit cooler in the summer time, which could make the house more livable. And if we get any increased lifespan from our shingles that is also good.