I may be dim, but I really don't know what to answer when google asks me for a URL when asking for a key for google maps.
I mean, I'm on my laptop, with no internet dns entry developing the solution in visual studio, so what do i tell google? localhost? The domain name I know the site will end up on? my laptops name?
I have always set up the API key to represent the sites future URL. I have seen no issues with using the API key on localhost that was setup for domainname.com, but that may be related to the subset of the Google Map APIs that I utilize. I would use your site's final URL when you set up the key for Google Maps.
Use the key which will correspond to the name of the server in the URL. I've used both localhost and the name of the laptop on local network.
Change to a different key on the production server.