Which symbol to use in my schematic for a supply signal which is also an output?
Input and output declarations typically imply signals -- be they analog or digital.  Examples include the input/output of a logic gate, an audio amplifier, or a microcontroller.  Power nets (e.g. +15V, +Vcc, -Vss, etc.) are typically depicted as isolated net labels or rails -- horizontal lines above (for positive) or below (for negative) the circuit:





I would suggest using this notation in your schematics -- the second circle in your image.  That way it may be easier for you to read since you can quickly discern signals from power rails, and it may be easier for others to read, too.