I never found it necessary to specify sinks. In fact, when I tried with sources and sinks set up, flow direction was indeterminant in most places. I set the circuit breakers in the substations as the source points and flow worked well from there. Of course, now with PV sources popping up all over, "flow direction" can only be viewed as "away from the substation", not necessarily actual electrical flow. GRU has some circuits with more PV generation than load at certain times of the day.
To me either you use Feeder Manager OR you use ESRI's way of specifying flow direction which is either done by specifying sinks OR sources. Specifying sinks, all current will flow towards the sinks. Specifying sources, all current will flow away from the source points.
If you have setup feeder manager, you would have to specify that you would like to use Feeder Manager to calculate the flow. This is done using the ESRI Utility Network toolbar, Options...there is a check mark...Flow -> Options.
Thanks for your input!
ESRI has a kind of long but really helpful video on geometric networks here, that briefly touches on this at some point:
Sorry, but I don't remember how far in they get to it. Basically, you almost never want to specify both sources AND sinks in the network, just one or the other, otherwise certain parts of the network may end up in a strange, indeterminate state, just as Kevin noticed, though I didn't realize how much of the network could end up that way. Also, performance is expected to take a big hit in that scenario (both sources and sinks).
Which one to choose (or whether to choose any) depends on the type of utility network, but it seems our electric customers almost always choose to specify the sources when they do. Typically there is only one type of device acting as a source, as opposed to any number of features (and a great quantity of them) that can act as sinks. Also, a Feeder Manager network is always going to have a source (i.e. an object with the model name DYNAMICPROTECTIVEDEVICE) on the map, while not every line segment will be terminated with a sink, so I think you end up with a more thorough coverage of flow direction going that route.
All that is based on advice for ArcGIS Geometric Networks though, and doesn't consider newer developments like the metering you mentioned, or all the PV sources that Kevin did, that are more a consideration with a real electric network these days. As it is now Feeder Manager OOTB isn't designed to account for either of those wrinkles in the utility model.