Get Woocommerce to Manage Stock as a default

Although this is quite late, since you asked about a hook: although there doesn't appear to be a hook, action or function specifically for turning on the Manage Stock option, you can still hook into the post save function and auto-enable it that way, since it is just a post_meta value:

add_action('save_post', 'myWoo_savePost', 10, 2);

function myWoo_savePost($postID, $post) {
    if (isset($post->post_type) && $post->post_type == 'product') {

        update_post_meta($post->ID, '_manage_stock', 'yes');
    }
}

Note that stock levels will always default to 0, so you may also want to add the line:

update_post_meta($post->ID, '_stock', '1');

...which will update your stock quantity to 1. Do be aware that this will happen every time a product is saved, though.

I'm unsure as to whether this has knock-on effects elsewhere in WooCommerce for larger stock quantities, but as you're dealing with single-quantity items, I'd guess you're probably okay.

Update (with $update):

As of Wordpress 3.7, a third parameter was added to save_post so you can easily tell if it's a new post being created or an existing post being updated. As such, you can fire the function above only when creating a new post (which is arguably the desired effect):

add_action('save_post_product', 'myWoo_savePost', 10, 3);

function myWoo_savePost($postID, $post, $update) {
    if (!$update) {
        //  $update is false if we're creating a new post
        update_post_meta($post->ID, '_manage_stock', 'yes');
        update_post_meta($post->ID, '_stock', '1');
    }
}

(Thanks to Dylan for the reminder about post-type specific saves)

A little late, but here's how for anyone else needing to do this... I found most of this code somewhere else, but I don't have the link anymore to where I found it. Tweaked it a little and this is what I ended up with (add to functions.php):

add_action( 'admin_enqueue_scripts', 'wc_default_variation_stock_quantity' );
function wc_default_variation_stock_quantity() {
  global $pagenow, $woocommerce;

  $default_stock_quantity = 1;
  $screen = get_current_screen();

  if ( ( $pagenow == 'post-new.php' || $pagenow == 'post.php' || $pagenow == 'edit.php' ) && $screen->post_type == 'product' ) {

    ?>
<!-- uncomment this if jquery if it hasn't been included
    <script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.2/jquery.min.js"></script>
-->
    <script type="text/javascript">
    jQuery(document).ready(function(){
        if ( !jQuery( '#_manage_stock' ).attr('checked') ) {
          jQuery( '#_manage_stock' ).attr('checked', 'checked');
        }
        if ( '' === jQuery( '#_stock' ).val() ) {
          jQuery( '#_stock' ).val(<?php echo $default_stock_quantity; ?>);
        }
    });
    </script>
    <?php
  }
}