Hi Albert,
The orders/fulfilled webhook will be sent every time an order is fulfilled. That could be valuable information depending on whether or not your application data depends on that knowledge.
Webhooks can be created and tested from your shop admin. Check out the manual here: http://docs.shopify.com/manual/settings/notifications/webhooks.
Store owners will receive and process orders just as any others, yes (though the caveats you mention still apply).
One thing to keep in mind is that even though Webhooks are a great way to get close to real-time updates, it's still a good idea to 'poll' our API occasionally to ensure your data is consistent. That way if a Webhook doesn't trigger an update for whatever reason (an error, Internet connectivity issues), the data will be made consistent.
Nate