I usually apply a thin crumb coat to the sides when doing stars, to give them something to stick to. Otherwise, they tend to pull off the side of the cake. That could just be me though. :0
Either way works. I have not had any problems doing stars directly on the cake.
most of the time I just do the starts and they have always stuck fine. Something to think about would be how thick of a layer of icing do you want there to be b/c the stars will only make a thin layer.
I apply a crumb coat as well simply because if I have a few gaps in the stars here and there then the cake does not show through.
Definitely crumb coat because otherwise any little crack between the stars is letting the air get to the cake and start to dry it out. Also, you don't want any place where cake is showing through.
thanks everyone!!!! that helps alot!!!!!