It is not versioned.
I have several junction feature classes but none are sources or sinks because, of the tens of thousands of features, just about all of them can be a source or sink.
I took the Working with Geometric Networks for Utilities and according to the class I don't need to define sources and sinks to perform a trace task. I don't have sources or sinks in the personal geodatabase and it works fine.
Don't misinterpret what I'm trying to do, my client will not be doing any modeling with this network, the geometric network is mainly for tracing flow, creating maps, and querying data.