Agree with JoannB...Fondant should not be thinner than 1/8 - 1/4 inch. I used to roll mine too thin to make it go farther. When I finally quit doing that I had MUCH better looking cakes.
Technically, your buttercream should be almost as smooth as if you weren't going to cover with fondant.
Do you mind if I ask something further? On cakes that have been carved, how do you get the buttercream underneath smooth enough so the fondant is then smooth. My only experience was that even a crumb coat was rough with the crumbs. I thought you could put fondant on top of that "crummy" crumb coat but I was wrong. The fondant was cellulity (is that a word 
) Do you know what I mean?
JoAnnB, thanks for that link. I just watched it and it was very helpful.
It is a bit easier to get a smooth crumb coat if the cake is cold and your icing is fairly soft. I add a bit a liquid to part of my icing for the crumb coat and it seems to work well. And of course, practice.
edited to add: don't apply a final icing or fondant to a cold cake. The icing and/or the fondant can pull away as the cake warms. let the crumb coated cake return to room temp before you cover it.
For my carved cakes I always firm them in the freezer for a little while before putting the crumb coat on. This makes a big difference. It is then easier to get it smooth prior to covering with fondant. A little bit of corn syrup helps to thin out the BC to make it easier to spread, but I don't do that under fondant, the extra moisture concerns me. I don't know that it would be a problem, I've just never chanced it. Not rolling the fondant too thin is the best advice.
On most carved cakes I don't crumb coat with a spatula...or at all. I just use the cake icer tip to apply the BC, which elimates the crumb problem. Then I smooth it out and remove some of the excess BC so it's not too thick. Also, on more intricate carved designs I use a large basketweave tip to apply BC intead of the big cake icer tip...
Courtney
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