I did some research and found that many experts say keep 'em charged because they self-discharge. But like all internet research, the answer you get depends on the question you ask. So now I don't know what to believe.
I will say this: although anecdotes are not evidence, the real-life testimonials from folks who use lots of batteries all the time are pretty persuasive. So, if someone says "we have a fleet of farm vehicles and we never keep the batteries on chargers during the winter," that's an opinion worth taking seriously.