Generally speaking, if a site would fit nicely into Main_Topic/Subtopic/Minor_Topic/, but the Minor_Topic category does not exist, then it should be submitted to Main_Topic/Subtopic/ instead. The editors there can then decide whether a 'Minor_Topic' subcategory needs to be created, or (as will normally be the case) the site can just be listed where it was submitted.
If there were enough sites to warrant making a category specifically for it, it would go under http://dmoz.org/Computers/Software/Spreadsheets/ and would be @linked from the StarOffice suite category.