The Utilities/Check/Repair options are wonderful in their own rite. However, if you have created new templates and/or events, there does seem to be some problems that can occur. If you made changes to the Event or role templates, it's safe to select the "Add New Event/role Types" and the "Replace Missing Event/Role options." However ... if you made changes to the source templates and then choose to Rebuild Source types, any changes you made will be overwritten. The source option actually rebuilds the source template library.
I'm not sure if you could restore just the event template library from a squeeze or not. Might be able to.