To tell you the truth, I have no idea what the defaults are (or even if there are defaults), as I think it'd be a bad idea to rely upon them. I always explicitly state what content the tag is expecting.
Additionally, if you auto-generate documentation from the TLD files (which I do, fastidiously using the <description> tags throughout the TLDs) omitting such information makes the docuementation much less useful.
If you do not auto-generate docs, being explicit within the TLD is even more important as it probably serves as the only documentation for your tags.