Slashes are added to input to escape values because that is how your server is configured. If you access the raw values of $_POST then you must strip them before using them to query the db.
You are not inserting anything into the DB, you are looking for something that is already there. In addition to this WP sanitizes the values before when using WP_Query. This would only be a concern if you were directly querying the DB instead of using WP_Query. This is what the prepare() method of wpdb is for.
