I have not seen anything official with regards to Java 8. This KB article for FM Server 13 mentions the recommended version of Java 1.70_67. I tend to get confused about this, but don't think the regular FM client app needs Java at all - unless you are running one of the 360Works plugins.
If you don't have to, don't update Java. FileMaker Server relies heavily on Java (though the admin console doesn't, thank God) so the last place you want to early update Java is the server.
You can take a test machine, make the update and see what happens after a few weeks. If nothing looks broken then you can plan a massive deployment.
Just my 2¢ of mexican pesos.
I received this, this morning, from 360Works support, in response to a question that I'd asked of them regarding Java and their plugins:
We are working on making sure that all of our plugins will work with 64-bit Java (obviously given the FileMaker technology being used is 64-bit) on OS X. Unfortunately, they will all still require the presence of a 32-bit Java 6 from Apple because of an Oracle bug. For some reason, Oracle's implementation still looks at Apple's Java Framework when natively starting a Java Virtual Machine (there are hooks present on any Mac even if you don't have Java installed). If the 32-bit version is not present, then the JVM does not load. If the 32-bit version is present, the 64-bit version is actually the one that gets loaded.
There is already a bug filed on this issue with Oracle, and I have filed a similar bug with Apple in hopes that there is some resolution in the future.
I'm hoping that this is resolved soon, becuase I don't like having versions of Java that are no longer supported running on my machine.