Most NiMH batteries `self discharge' in a month or two. Expect 75% of the charge to be lost in 30 days -- even if the batteries are completely disconnected. This does not damage the batteries, it just means you can't rely on them for a lot of power after a few weeks.

With conservative design or a different technology battery somewhere for bootstrapping, cars like the Prius could probably survive a couple months of disuse without an external charge. It seems likely that letting the batteries discharge completely shouldn't cause damage, even if an external charge is required.

You can easily test the NiMH discharge rate with batteries from the drug store. Charge up a pair and put them in a flashlight and test it. Put it on the shelf and let it sit for a month or so and test again ...

There are some very new NiMH batteries on the market now that don't have the self-discharge problem. Eneloop and Uniross Hybrio are examples. They come ready to use, no initial charge necessary.

Richard