It is always OK to use a larger ESC than you need. As Ceejay said it gives you a safety margin. It's never a good idea to run any component right at the limits of its specification.

The only penalty you pay for using a larger-than-needed ESC is that the larger ESC will usually be slightly heavier than a smaller one. So your plane will be carrying a little bit of extra weight. That's not usually a problem for most planes.

- Jeff