the latter, e.g. I'd assume an competent ISP with a large customer presence on the west coast would have DNS servers for their customers on the west coast, or just use anycast...

Yeah, not sure. I think most ISPs use just a couple of DNS servers for their entire network -- or at least that's the case here in the UK. But as you say, anycast might mean that those two IP addresses are actually represented by many servers.

I guess we'll need the input of someone who works at an ISP :)

"A couple DNS servers for their entire network" in the UK is equivalent to having geolocated servers in North America.

The UK has a small land area, and so latency talking to the other side of the continent is nowhere near as much an issue.

You would hope that to be the case, but it is not generally true.