I was Googling around for information about battery storage, in particular the long-term storage of Ni-MH and Ni-Cad batteries, and I found this Battery Storage FAQ from PowerStream It covers most types of batteries you're likely to run into, not just Ni-MH and Ni-Cads. It says to store both types discharged and to charge before using.
Of course, I've been around the Internet enough to know you that you must confirm information. So I kept going down my search results. A number of sites say that Ni-MH batteries should be recharged regularly during storage, and should never be allowed to fully self-discharge. (For those who aren't familiar with the characteristics of the Ni-MH batteries, they loose charge at around 10% per month of storage. They also have a variable, but finite, number of charge/discharge cycles, usually quoted as 500.)
Other sites, such as Duracell and Energizer, say that the batteries will self-discharge to zero over time and must be recharged prior to use. They also note that it will take about 3 charge/discharge cycles before the batteries will hold a full charge again. they don't indicate that there is any difficulties with long-term storage of discharged batteries.
But another battery pack manufacturer, Harding Energy, agrees with PowerStream. Store 'em discharged.
So what's a poor boy to do?
I have some Ni-MH batteries that have been stored since the early spring. They were stored charged, and should be fully discharged by now. I have some Ni-Cads that have been stored a similar period of time. We're going to charge them up and see how they perform. I don't have a lot of really sophisticated measuring equipment, but I should be able to reach some useful conclusion.
Stay tuned for details.