Chairs or Chair

Arguments:

While it's probably more intuitive to name things using singular forms (e.g. woodworking drill) when you're cataloguing, some words don't have a singular form e.g. scissors, trousers. Many terms are collective (e.g. equipment, furniture) and many things come in pairs, e.g. shoes. When you're searching, it may be more intuitive to use plurals, (e.g. I'm looking for woodworking drills).

Alternatively, it may seem more intuitive that proper nouns (which would populate role/occupation/activity fields and locality in the Lexicon) are normally singular in form (eg. George Washington, not George Washingtons), so all terms should take the singular form. This decision really only applies to fields where the terms are all nouns, e.g. Object Name. Proper nouns don't have a plural and will appear as normal, e.g. George Washington, Paris, Eiffel Tower. Subjects need to have consistent grammatical form, but won't often be plurals e.g. feminism, underwater transport, gardening.

Clearly a plural or singular preference is preferred, but if you must include both singular and plural terms in the database, it is important to add every singular term for the plural term and vice versa. This will result in a lexicon with twice as many terms, but will enable all searchers to find all relevant records.

This is a realm where reasonable people might have to agree to disagree and the biggest dog (cataloguer, scholor, or politician) will rule. Whichever form you choose can be a Primary term while the other term can be designated as an Alternate term. The most important standard to embrace here is consistency. Whether you choose plural, singular, or even both with one designated as primary, this standard should be followed for all non-Proper nouns.

Z39.19 also deals with this issue.