As Mily said, you can attach molded fondant pieces either way. When I don't want to mess up the shape of something by handling it and attaching it to the cookie, I'll wait for it to fully dry. BUT, if I need the piece to conform/fit to the cookie, I always apply the piece soft. I agree with Mily - I think modeling chocolate tastes better though. Also, different brands/types of fondant dry very differently. Those with a lot of tylose powder in them behave more like gumpaste and dry very fast to a rigid state (like, Satin Ice); others can be much softer (and are far better for wrapping cakes than Satin Ice) and almost never fully dry. Just something of which to be aware.