This has probably been a million times but I can't find a definitive answer.
We are currently building/releasing our app with Xcode 6.4. We plan to release next week finishing testing this week. At this time though Xcode 7.0 will also be release, which adds a couple other changes for us. Is it acceptable to continue releasing the app using Xcode 6.4, even if for a week its not the latest version?
If the past is any indication, you can continue to use slightly older Xcode versions when a new version is released. Note that the minimum at this moment is Xcode 5.1.1 (logical SDK requirements still apply).
Cool thanks KMT, what do you mean by Logical SDK Requirements still apply? And what are you going by to say Xcode 5.1.1 is the minimum?
As an example, Xcode 6.4 can't support iOS 9.x, so logically, that means you'd need to accept a lower iOS when using it to build for the store. There were several different SDKs used from Xcode 5.1.1 up to now, so the version of Xcode you use will mandate which SDK and which iOS version(s) you can expect to support.
As for Xcode 5.1.1, it's common knowledge that when submitting an app to the store, you can expect to see a dialog alerting to which version Xcode is currently allowed as minimum. At this time, that dialog warns the dev to use Xcode 5.1.1 or higher.
