I find that I have spare batteries in virtually every location and kit, so I don't tend to replace the bat's in a flashlight until they seem dim, with one exception...

My EDC light is a single AAA light on my keychain. Right now it's a Fenix L0D, but, it was an ARC AAA for a while, probably a Mag solitaire before that. I replace that battery with a fresh one fairly often, probably monthly... because when all I have is my keychain, I don't have a spare battery, so I think it's worth doing pre-emptive battery changes.

Usually, the one I'm taking out is fairly fresh, so often use it for something less critical that uses AAA batteries, and put a fresh one in the EDC light.

On a related note, for my EDC light, I spend the extra money for the AAA Lithium batteries. Most everything else gets more conventional Duracells. Do you folks find that you use Lithiums for more things now that they are available readily in AA and AAA?