The programmer needs to know the function of the circuit, the direction should be obvious from that. Eg. an LED is an output, a switch is probably an input.
As a programmer, I've looked at many schematics, and would never rely on the labels, they are too often wrong. The CAD does not check the labels, and the electrons unfortunately do not know about the label
To help the programmer, I would suggest using a scheme such as type_name_direction. For example BTN_STANDBY_IN or LED_STANDBY_OUT. This was from a real example, where the circuits were labelled something like PA13_STANDBY and PA17_STANDBY.