The rule of thumb is to use deviation-based scores (like percentiles usually are) if you want to compare performance to a group, or performance on different tasks to each other. Raw scores generally cannot be compared between any two statistically unrelated tests. The rapid change in percentiles to either side of the 50th %ile is because of the bell curve. The significance of the difference in percentiles is also relatively modest between, say, the 35th and 65th %ile, due to the "bunchiness" of the population around the mean.
