The DMG p. 238 states: a saving throw is “almost never done by choice…a saving throw is a split-second response to the activity of someone or something else.”

Almost never is not “no such thing” so there are potential times someone can choose to (or not to?) make a saving throw. I haven’t seen RAW list these so it might just be up to interpretation.

A saving throw throw is a response to something/one not one’s self.

So the specific of Modify Memory could over rule the general of not having a choice.
Calling for a saving throw against one’s self might fall under the category of not needing to make a roll if the outcome is not in question.