It's interesting, because despite devs having stated that the number of souls you summon at one time (so, stacks vs individual) doesn't affect what toon is summoned, it seems that the 4* and 5* toons tend to appear in stacks more often. At least that's what I find anyway. I prefer saving souls right up until the next sac event, as I usually don't have enough space and like having as many toons to sac as possible at the time of the event. I've even gotten into the habit of saving frags until sac events so that I might get a newer toon, since I'm no sure when a toon is assigned to a soul.