I think it also depends on what kind of warranty you have on the batteries. For example, I purchased an extra battery with my Janty Dura (for a total of 3 batteries). I rotate all three of them because I have a 3 month warranty. I want to use them all so that if I have a problem, hopefully, I'll figure it out by using them fairly steadily during the warranty period.
Now, for batteries with a short warranty period, like a couple of weeks, I'll use it a couple of times to make sure there's no immediate issue with it, then it goes into a drawer to be pulled out once or twice a week.