I don't think you can unroot again. When you will root your device you must have to flash some recovery like CWM recovery which cannot be removed. But if you will root by another method you can probably unroot your device.
Yes, unroot it if you do want for the warranty. But root your smartphone is also a process which will change some original data and i'm afraid you have to update your ROM. Actually the rooting app can finish that. I had an experience.
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