You made a good point by comparing the self discharge rate of nimhs to circuit power consumption. There are also "low selfdischarge" nimhs on the market, for example panasonic/sanoy eneloops. Specs give them discharge rate of 5% per year.
I try to achieve 1-4 month battery life for the high consumption nodes. These nodes will spend most of the time sleeping, but when action is requested, they consume a lot in short time frame.
You still made a good point, that I didn't consider much before. I will re-evaluate what is actually relevant with power optimization. Thanks :)