There are multiple reports on how wp-cron is a far from ideal solution because it runs every time a page is loaded, which is unnecessary in most scenarios (one scenario in which it would be necessary is when you use scheduled posts)
The common advice is to add
define('DISABLE_WP_CRON', true); to wp-config.php then schedule a real cron job (if you have enough admin access to so).
But as of WP 3.3, there is WP_CRON_LOCK_TIMEOUT, which "defines a period of time in which only one cronjob will be fired".
If you run hundreds of WP installs you would have to create (and delete, when that wp install is removed) a LOT of cron jobs, which can become a nuisance. Therefore, it seems that the best solution these days is setting
define('WP_CRON_LOCK_TIMEOUT', 900); (if you want wp-cron to run every 900 seconds).
The question is: has anyone used WP_CRON_LOCK_TIMEOUT with this purpose yet? Is this its intended use?