If you have a more powerful computer, you may leave the quality at high, though if you have a laptop or an old computer, it might be best to set it at low quality. Another note is that when you have high quality you are more likely to get lags. You should use low quality for regular play and high for speed plays.
There is not a large graphics difference in-game.
The High Quality image on the menu has good resolution, while the Low Quality one on the menu is blurry.