I have learned a lesson with frosting a cake while frozen if using crusting buttercream. If you want to smooth out cake with crusting buttercream then wait til thawed out.....
I wanted to see what would happen if I iced a frozen cake. basic laws of science apply! Cold contracts and warm expands. I iced the cake while frozen, then as it thawed (i.e. "warmed") the cake expanded and the BC icing cracked all over. Fortunately, this was for a family pitch-in, so with the chantilly lace decor that I added and some BC flowers, it looked ok (to everyone else). 
I have crumb-coated while frozen, then let it come to room temperature before doing the final icing and those turned out ok.
i have never frosted a buttercream cream cake while it has been fozen, but when i make a cake with pastry pride (non dairy whipped topping) i always have it forzen it keeps the cream cool and from craking, i also don't have to clear room in my refigerator
frozen is the only way i'll ice the cake....to many crumbs on a thawed cake...
although i've known the heartbreak of the cracked icing if the cakes frozen solid...i usually just freeze overnight.
I never decorate a cake unless it is frozen and i havent come across any problems with cracking or the icing not being smooth.
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