I pretty much exclusively decorate with fondant, and I've NEVER had a problem with cakes not tasting "fresh" days and days later. Right now I'm making a 3-D "Cars" cake for a friend's kid's birthday, and I baked the cake on Monday, sculpted the car and covered it in fondant on Wednesday, and finished decorating it last night so that I would have time for my other cakes this week that aren't as detailed. She won't be picking it up until Saturday at noon. I guarantee that cake will still be awesome at the party Saturday evening.

I'm not going to charge someone for a cake that tastes like a$$. That's just not how I operate!