Minor releases, or service releases as we call them, may introduce newer API. APIs otherwise are consistent for the duration of the whole major version cycle.
Behavior may change if bugs get fixed in code fronted by the API, but the plug-in should continue to work just fine. You should use the oldest possible service release where necessary, but have users of your plug-in update to newer when issues in Rhino get fixed. Especially when you find that API that you use has received fixes your plug-in relies on. The minimum version you build against is what the users of your plug-in will have to install, or later.