yes, you should remind your helper from time to time that she should be honest and not to take away anything in house without your permission. You should store your valuables away as if maid is not good, it is just like inviting thieves to your house with your door open.
tli, I don't let my maid to touch my crystal. If you have a locked glass cabinet, lock it. All the cleaning of the crystal is done by myself.
Not only that there is a risk of loss, there is also a risk of broken. If the maid breaks your crystal, she only need to pay you back $300 per item as compensation...