Excellent answer Brent, can't go wrong with that.
Not all systems CAN use a return spring. Internal throw out bearing comes to mind, some cable systems could be problematic as well.
Personally I can't think of a single "fork" system with an external slave that couldn't benefit from a return spring. The slave cylinder piston MUST be totally bottomed out to get the proper adjustment. Some systems kind of sort of "self adjust" more than likely you got "lucky". There is only way to be sure, gotta spring it.