Love this app but have it set to Location Services only while using. Otherwise your battery go bye bye.
I don't feel it kills me too badly. This is one of the main appeals of Dark Sky -- you get very timely reports on rain and other conditions starting/stopping wherever you are located. It's pretty much spot-on, I find. Weirdly so. And again, my battery life is pretty good with it running in background always. This is an iPhone X with original battery purchased when they were released.