Do you have to...？可以用No, I needn't.吗

口语怎么用都没问题，但是如果是考试题的话 这样用很有肯定是要扣分的。
因为have to 和need 虽然都是需要的意思，但是have to 是强制的意思 也就是说 不但是需要 而且是必须要 而need 只是自己的愿望 需要而已可做可不做可有可无，所以 NO后边最好跟 have to 而不是need

可以
美剧对话中这样用到过

不过I needn't不是前者的通用答语，只是在某些特定语境中可以这样用

譬如：
A:Do you really have to tell him all about this?
B:No, I needn't. But I think it's better to tell him the truth. It's good for both of us.

我更愿意用 no, i don't have to.
但我也没法说你这个不可以

yes, of course.

Yes,I do/No,I do not.