I was mostly looking at https://wiki.openstreetmap.org/wiki/Key:priority: 

> oneway=alternating - When the narrow section is controlled by traffic signals instead of priority signs

This strongly suggests it should not be used when there is no traffic light.

Looking at https://wiki.openstreetmap.org/wiki/Tag:oneway=alternating I'm uncertain. Some of the wording seems more geared towards a controlled traffic flow like a traffic light. For example, "This tag is only used for roads that change direction often (usually several times per hour) and regularly." With signs there is usually not much regularity to the traffic flow and mentioning some sort of time interval seems odd. Also, "Routing engines are encouraged to route over this type of oneway independent of direction but use a heavy penalty when calculating the travel time to allow for possible waiting times" seems not fully appropriate for a simple sign.  Perhaps, this is something that should be discussed with the community. The wording certainly could be clearer.