Best Answer:
If using modern Alkaline batteries, that depends on the manufacturer. If you only use your flash once per week, and you are using store-brand or Duracell batteries, I would remove the batteries. Duracell batteries, as well as any of the high capacity alkaline cells, love to leak. I'm trying out Panasonic batteries (lower capacity than Duracell) for things that sit around for a couple years and haven't had any leakage issues, however, the experiment is only two years old.
If using good quality NiMH batteries, they seem to be very robust. I haven't had any leak on me in the past 10 years. I leave my NiMH batteries in my flash and flashlights without worry.
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