I'm reading recipes for Tiramisu that call for Zabaglione. Some say I can simply whip egg yolks with sugar in a stand mixer, while others insist I use a bain marie and that the mixture reaches a certain temperature (68 degrees Celsius). Why is it so? Is it for pasteurizing? If so, can I use my sous-vide to pasteurize the eggs (by setting 60 degrees) and then simply use my stand mixer on the counter, not a bain marie? If not, how does it affect the cooking? Thank you! 
It isn't about pasteurization, it is about texture. Egg yolks whip very differently at different temperatures. When you whip egg yolks with sugar at 60 C, you will get a very airy foam, which will also be more stable than a foam which has been whipped at room temperature. So, for optimal results, you should whip in a bain marie with a handheld mixer. It is not completely impossible to do it by hand, but you will need to whip for a very long time, until you have the perfect foam. If you want to skip the bain marie, then you can do so. You will simply have a slightly denser zabaione. I would be wary of trying to use yolks which have been pasteurized in a sous vide machine. The texture of a yolk which has been taken to 60 C without agitation is different than the texture of a yolk which has never been warmed up. If you want to pasteurize your yolks for food safety, and then go into the stand mixer, do so, but you will have to contend with a zabaione that is a bit worse in quality than if done with never-heated yolks. 