In reference to the ultrafire 18350 cells. Its possible they are ok. But, a couple of efests cost a tenner and are known to be good. Aw cells are known to be good. As long as you use a reputable, known vender who is active on the forums relating to li ion devices (torches, rc, now vaping etc) there is a very good chance you wont get (pardon the expression) burned, take a risk on an unknown ebay seller or budget store from china, or the cells are ridiculously cheap, id be very very wary.
As for stacking cells. Well, there is little need to do it in vaping, unless your building some sort of vv/vw device where the board needs 8.4v+ input to give full regulation. (Vamo springs to mind) in that case, reputable cells are more likely to "match" consistently and so discharge at an even rate, using a good reputable charger is more likely to result in balanced cells although the ideal would be to connect the cells up on series with a balance lead and use a hobby charger to charge.....yeah that's pretty involved, I'm content using my xtar wp2 ii for most charging duties, but I only regularly use series cells in my high out put torches I build, dont forget, you can get 20w out of one 4.2v cell with a dna20, stacking cells is just not necessary for vaping.
All the above information is also why the cheap 510 devices with a sealed cell and usb charger scare the crap out of me, I know enough to just not be prepared to trust those devices, not when I know I can run cells I trust and chargers I trust to terminate correctly.
