I don’t think there is a generic answer. it depends on your own preferences. after the import from evernote I have cleaned up most of the unfortunate groups that had only a single file with a link through @BLUEFROG 's script:
other imported groups, with more than one imported “attachment” (which were part of a single note in evernote) I am tackling as I re-encounter them:
some of them I convert into RTFs, others I am leaving as is, mostly depending the number of “attachments” they have. the more there are, the more likely I leave them as a group as I probably never should have put them into a single note to begin with.
what I did start doing in the same context though was using wikilinks and / or x-devonthink links to link those note to each other…