There are two reasons why I think you should have a few different controls available to you.
Firstly, it helps in spectator handling. There will, every now and then, be an awkward spectator who calls you on what your doing. If you always do the double undercut, they'll probably let it go the first time, but the second time you use it, they'll either directly accuse you or make it clear that they think you're cutting the card to the top. If, at that point, you could catch their eye and execute a pass, thus apparently continuing the trick without the cut, then the effect becomes more powerful than if you made some excuse for cutting the deck.
Secondly, it means you can be more creative. You don't always have to get your spectators to select and return cards in the same way, which opens up the possibility of varying your effects, improvising, and creating your own ones.