Yes, it is. The build system only accepts "Release", "Debug" and "DebugRelease" and builds a release and/or a debug flavour of the library. The mapping to the use flag is that +debug will install both libraries, release and debug, -debug will build and install only the release library. This is similar to the behaviour of boost and tbb.

I've enabled +debug per default because it is very common to develop code with deal.II with both libraries in place.