Hi Chris,
I hate to say it, but, "it depends" on a few factors. Typically, the rule of thumb I share with folks is that they need to republish & check the configuration of the web app/map after a schema change just in case. I have seen instances where changes to the service are not picked up by ArcGIS Online/Portal web map. I have also see instances where it isn't a problem. In either event, it is always good to CYA when using referenced data.
Likely this is not the answer you wanted or were seeking, but this is the local practice here typically.