When to use Reset Query

I am excluding posts in a particular category from pages that use the index.php as a template using query_posts(). It’s the only way I have found to exclude posts from a category that isn’t a plug-in and doesn’t mess up something else on my site.
Here’s the code I have:

<?php query_posts($query_string . '&cat=-3'); ?>
  <?php if (have_posts()) : ?>

My question is whether it is necessary to reset query after the loop, which would look like this:

<?php endif; ?>
   <?php wp_reset_query(); ?>

Is there a benefit or drawback to this? On a basic level I understand what the reset is doing, but as far as best practices – should the query be reset every time I use query_posts() (since I’m changing the state of the query)or is there a rule of when to use and when not to use?