You can do either, and I'd say the trade-offs are subtle:
- Readability: Is it easier for the reader to make sense of them in the steps or in the description?
- Intention: Are you describing a summary use case, where the list of steps is a summary of what use cases get performed? Do they need to be performed in a particular order?
- Diagramming: When related use cases are on the same diagram, an arrow will automatically be drawn between them. Do you want the arrow on the label to say "included" (then place them in the steps), or "id link" (then place them in the description).
- Reporting: similar to diagramming, do you want the use cases reported as included use cases? Note this only affects custom reports that use the IncludedUseCases keyword (IIRC, none of the built-in reports display included use cases).
Hope this helps. Let me know if you have questions.