Funny, but I just recently started overcoming my longstanding top-change anxiety by tacking a top-change sequence to the end of Ambitious Card, much as described above.
The way I do Ambitious Card, I give the deck a "tap and a slap" each time I make the card rise to the top. After a couple of rises, I hold out the deck and have a spectator tap and slap; when the card rises, I ask how long she's been studying magic. (Best response so far: "Oh, for about the last 30 seconds.")
We repeat, and when the assistant succeeds again, I take the card in my right hand, look at her, and say, "Hey, you really are a magician aren't you?" Then I give a paranoid look at everyone present and say, "Wait a minute ... you're ALL magicians. I can tell. I've stumbled on a magic convention or something, right? God, I hate performing for magicians!" I pick out someone slightly to my right, pivot my body, and say, "You, you've got 'magician' written all over you!" The top change takes place as I thrust my left hand toward that person, pointing accusatorily. I then conclude with an especially triumphant final rise.
Apropos of Nathan Coe Marsh's well-expressed comments, this top-change sequence involves triple redundancy:
1. My impeccable (heh) technique;
2. The misdirection accorded by the spectator byplay; and
3. Synchronization of the sleight with the word "magician," to mask any possible talking of the cards (on the off chance that my technique proves, er, peccable).