Strongly connected components on a DAG
I know of no convention; the right result depends entirely on what you want to do with the result.  As an example, if you're trying to use strongly connected components to find satisfying assignments for 2SAT instances, then you likely want all the singleton cycles returned along with the larger components.  But if you're just trying to find out if a graph contains a cycle, then the singletons won't be of interest to the caller so the SCC function could usefully omit returning them.