Thanks for the info, Poeia. I just saw a video on li-ion batteries and the guy said your pv is no more likely to explode than your cell phone or laptop, which also use the same types of batteries.
Stacking batteries in flashlights never concerned me, but I guess that's because they're not li-ion batteries. I'm becoming more worried about this whole stacking batteries thing than I probably should be and I know that these companies warn against it to protect themselves. But I use so many mods on a daily basis that I can't remember which battery was on top last time and in which mod. I do keep them in pairs, but I don't bother with putting the battery that was on top last time on the bottom the next time. I guess I should start keeping logs or something, lol.
Li-ion batteries in laptops can (and have) exploded. I've heard at least one resulted in burning a house down.
Li-ion batteries in PVs can (and have) exploded (and I'm not counting the bozo who recharged non-rechargeable batteries.)
But if you consider the number of laptop batteries in use every day and the number of sets of Li-ion batts we pop into our PVs, the odds of it happening are very, very slim. Take some simple precautions and you can reduce the odds further -- no unprotected li-ion batts (IMRs don't need it for whatever reason that I've never bothered to fully grasp); don't leave your batteries charging while you are asleep or not at home, etc. If you're more concerned, I guess you can always pop your batteries out of your PV before you go beddy-bye.
I never heard anything about which battery goes on top but, if you are worried about that, get a Sharpie and label one battery from each pair "T" (top) and one "B" (bottom.) Of course, this is the Ovale V8 thread and, as its battery layout is side-by-side, we don't care about that here.
If you think flashlights don't use li-ion batteries, go read CPF -- the candle power forum. Compared to the flashlight freaks, we vapaholics are running our PVs on static electricity.