It's optional. There are only a couple of places where plurals are either required or restricted, and this isn't one of those places.
(If you're interested: the plural suffix is required if you want to say Hoch Dochmey all things instead of Hoch Doch each thing. The plural suffix makes the difference between considering each thing individually or all of them as a group. The plural is not allowed when asking how much: Doch 'ar how many things? You cannot say Dochmey 'ar. Plural suffixes can't be used normally on nouns with inherently plural forms without taking on a scattered all about meaning: jengva' plate, ngop plates, jengva'mey plates scattered all about, and you can't add a plural suffix to an inherently plural noun; ngopmey plateses is ungrammatical.)