I answer you as a Spanish speaker: if it's understood that you are talking to "ustedes", saying it or not saying it seems optional to me, and I probably wouldn't say it. If it's not clear that you are talking to "ustedes" and it could be confused with "ellos" (that has the same verbal form) I would say it.

Hola! I agree with Nadja. That is optional and as long as I know it, you don't have to put it out if you don't want. Therefore my advice would be: choose the way you feel more comfortable and don't worry about it any more. Saludos de España!!

The verb determines if you need usted or ustedes Usted cena muy tarde. Ustedes cenan muy tarde.

'Usted/Ustedes" se utiliza para utilizar un tono formal o cuando no tienes confianza con alguien y quieres ser respetuoso. Por ejemplo si vas a comprar a una tienda normalmente te hablan de usted

¿Ustedes cenan cuando vuelven a casa? it is difficult to explain but if you are writting and haven't mentioned in other phrases before in the paragraph the noun ''Ustedes'' has to be written but if you are speaking you can chose to omit and it will be infered if you are directing yourself to a group of two or more people. For only one person you can use "Usted" is the polite or more formal way of saying "Tu" and is a way to show respect for the person you are speaking to. ¿Usted cena cuando vuelve a casa? if you notice, the words "cena" and "vuelve" has no "n" at the end if you finish verbs with an "n" that will give the context that you are speaking about more than one person.