I voted "dont care", but I stack batteries only on mods that are done for that (mostly vv devices such as the Vmax or else)
Talking about one particular mod, one reseller told me once it was better to use one 18650 battery in it than stacked batteries, and that's what I do on it, so to be as exact in my answer as possible, I'd say : in some devices I'm using stacked batteries when indicated in the user's guideline, otherwise I use one single battery, I always do what's recommended as I don't want the device to blow out in my face.
(that is to say "don't care" is unappropriate here in the poll, but I hadn't any other box to check than this one.)