Don't even think twice. If you're not using it TODAY, go to your flash (and other electonic gizmos) right now and DUMP THOSE DOUBLE-A's on your desk. Storing your equipment, even for a modest amount of time, with the batteries installed is NOT a good idea. Batteries leak, and the result is a mess--corroded contacts at the very least, and you can imagine where it goes from there.
How do I know this? Funny you should ask. I don't really want to talk about it.
The other day I pulled out my Nikon SB-800 Speedlight to do a little experimenting with a new technique. No power. On top of that, the door to the battery compartment wouldn't open. At that point battery meltdown was the furthest thing from my mind. The unit was in clean, pristine shape--no sign of trouble...
...except for that door.
Not wanting to force it open and damage it, I did a little web search to try to figure out why the door wouldn't open. That was fruitless, of course (no nitwit could possibly fail to open the door!). Then a light popped on--the freakin' batteries!
I'm pleased to say that I didn't explode. The damage was undoubtedly already done. I got a butter knife and pried the cover loose. Sure enough my Duracell AA's had committed sepaku, spilling their guts all over the inside of the battery compartment and fusing the door shut with their blue-green entrails. The contacts were shot and I had a repair job on my hands.
It strikes me as ironic that batteries from the same purchase, stored OUTSIDE of equipment in the camera bag are just fine. It's almost as if they turn into little monsters bent on destruction when they're installed.
Nikon was more than happy to have me ship the flash in for repair. Thankfully, I could not hear any chuckling to themselves as they explained the process--and that it would likely cost more than $100.00 to fix the damage done by four bucks worth of coppertops.
SO...don't do this. Keep your batteries outside your electronics until you need to use them. When you're done using the equipment, take the batteries back out.
That's your tip for the day. I just saved you a lot of money.