JAMB - English Language (2011 - No. 44)

When there is no special emphasis, the rule of thumb often applies that a positive sentence has a negative tag and vice versa. This form may express confidence, or seek confirmation of the asker's opinion or belief.

For example: It's cold (positive), isn't it (negative)? just like in the question above,

''Suara needn't (negative) come with us'' Need she? (positive) correlates with the question tag