I recently did a 6 0 cake, and both the holes in the 6 and the 0 were awkward to get fondant in, so they became a bit of a patch job, with well-placed and creative decor covering the rest.
I had originally thought that when I laid the fondant over, I could just cut an "x" in the fondant where the holes were and work the fondant into the holes, which kinda worked, but not really. I rolled the fondant out at my usual thickness (1/8" or so), do I don't know if thicker or thinner would have made any difference.
I'm sure someone with more experience would have less difficulty with that.
It was for a friend's birthday, and he, nor anyone else there, noticed anything. They were too busy looking at the rest of the cake.