John Irving and Stephen King have publicly weighed in on the rumor, oddly enough. Both have warned her against killing off Harry Potter by citing complications that arouse for Sir Arthur Conan Doyle when he gave the axe to Sherlock Holmes.
Rowling gave a rebut citing that both advising authors have killed off characters in their respective books, but, to be fair, both King and Irving write completely different types of fiction than her.
As far as whether or not it is a good idea to kill off Harry Potter, I think it makes sense if a case was furthered that she has really been building this fictional kid up to be the ultimate martyr. How many times did she beat over the heads of the readers that he was the chosen one....that he was the gifted one? Logically, the messiah must make the largest sacrifice in the end of an epic: a concept that science fiction and fantasy fans alike are quite familiar with.
To be honest (a phrase almost exclusively meant to forewarn the reader of a less-than-nice comment), I have never thought the series to be a good as the hype that has long surrounded it, and, by this point (which has been reached after past books have twisted the knife into the backs of other characters), I am not entirely convinced that the death of Potter wouldn't be an exercise in a final plot twist in a long line of shockers meant to keep the fans reading or from moving onto literature with more than one dimension. Potter's death would be a finale that would talked about months and months after the fact.
I have read that she may wish to kill him off in attempt to head off any possibility of another author continuing the tale after her death. It may be a lost cause. If Sherlock Homes can be brought back to life, any literary character is fair game.