Writing sample shows your writing credibility? How much you can write within an hour? Are you able to develop coherent ideas and able to synthesis in a coherent manner within an hour or not? As from what I have known T is the best letter grade while J is the lowest grade you can get on a writing sample. But, dont you fear about getting a low score on a writing sample, I think it comes into consideration when two applications with the same criteria are tied. Then the writing sample becomes the matter of sprinkling your application a little. Or else, dont worry too much about the writing sample letter grade.