I think a lot of theme development is getting away from using add_image_size because WordPress makes way to many images. So rather than doing it that way, you can use a plugin or even a plugin that will integrate into your theme. Here is a good one that is easy to use
I think this is a better way to go than doing the add_image_size. from the script, you just call whatever size and shape you will need.
For example. Lets say you need an image that is 125 x 125 which would give you a nice square. You could do something like this inside the loop.
$thumb = get_post_thumbnail_id();
$img_url = wp_get_attachment_url( $thumb,'full' ); //get full URL to image (use "large" or "medium" if the images too big)
$image = aq_resize( $img_url, 125, 125, true ); //resize & crop the image
<article <?php post_class()?> id="post-<?php the_ID(); ?>">
<?php if($image) : ?>
<img src="<?php echo $image ?>"/>
<?php endif; ?>
That would take the featured image of the post and crop the image into a 125 x 125 sqaure and echo out the image.
You can see more examples on this page