Return Taxonomy/Term Information with Posts (WP_Query/get_posts)

WordPress will cache all terms for taxonomies attached to all post types in the query result set by default – well, so long as you haven’t set either cache_results or update_post_term_cache to false (codex).

So calling get_the_terms etc. within the loop will not hit the database. However, wp_get_object_terms will hit the database as it by-passes WordPress’ object cache.