I think using the icing tip after crumb coating is a good idea. I'll have to try that next time since I tend to still get a few crumbs when putting on my 2nd layer of icing with the spatula. I also just want to mention that I made a devil's food cake for a bridal shower and I used Wilton's extra special buttercream recipe. It was fantastic with the devil's food cake! Check www.wilton.com
for the recipe if interested.