I use "David's Cull", or "A Brave New World". In the past I have used a slop shuffle to separate the cards.
I don't agree with David's above statements. For those of you who are new to the forums this is very rare. I always apreciate David's input. That said, OOTW may be my favorite plot and I have thought about lot about presenting it with maximum effect for lay audiences.
First, I believe that you should be able to get into out of this world from a shuffled deck. Why? Because in my view this effect is a closer. It might be my most requested effect, and leaves a lasting impression. This makes it very hard to follow. So to make it a closer you have 2 choices. Plan ahead and have a deck set up or get into it on the fly. I prefer to get into it on the fly. If we are playing cards as friends and someone asks to see something I can pull out the big guns with a moments notice.
Second, my personal presentation is to have the deck shuffled and shown to be mixed. I then set out to prove the the participant has "memorized" the cards. This works with the versions of the effect that I do. If your presentation is different I could see a justification not to shuffle.
Third, the veils principle as stated by Darwin Ortiz applies here. At least I believe it does. The participant making decisions is one veil, it partially obscures the method. Adding the shuffle adds another veil. Which obscures the method even more. I don't see harm in this. In fact I find the line "and you shuffled the deck!" to bump up the imposability a couple notches. Of course I concede that this thought process can lead to overly complicated effects and magic that only magicians enjoy.
It comes down to when do you want to be able to perform this? At the end of a routine? At a moments notice? And, what do you want the perceived method to be? That the participant is tapping into their subconscious memory? That they are really lucky? That you have some how placed the cards in specific places as predictions of where they would put the cards?