I guess it would be for gambling routine. I'm only trying to retain top one or four cards; deck switch is an overkill. Speaking of deck switch, do you know resources to learn them?
If it is only a few cards, why not just use an overhand shuffle, or even a riffle shuffle that keeps the top stock on top? Or, if you really absolutely need to give it a wash, cop or palm the cards off, and place them back on after the wash.
But for your deck switch:
The Art of Switching Decks by Roberto Giobii is a great book (and DVD I believe) on the subject(I don't own it, but have heard a lot about it).
I know Daniel Madison teaches 2 or 3 in his book (All of his books are compiled into his new one) "Devilry"
You might also want to look into a deck switching Gimmick, instead your standard deck switch. Here is a list of some of the one's I own/know about.
Switcheroo by Michael O'brien
Cold Case by Gregory Wilson/Fracture by Daniel Madison (Fracture is a DIY gimmick from Madison's book, Cold Case is a great gimmick with the same principal, but is pre-made, and super durable).
Flash Deck Switch 2.0 by Shin Lim (Takes A LOT of practice, and only usable in some scenarios).
Cooler by Christian Engbloom (Only usable in some scenarios)
Now, I don't know if these will necessarily work in a gambling routine scenario. Of these five, all of them
except for Switcheroo utilize a "box" gimmick - that is, a gimmick that looks like (or is made from) a tuck case. Any tuck case (for any deck) can be applied to the gimmicks (except I don't think you can customize Cold Case very much, but you can with fracture. Gregory Wilson might go over how to customize it or make your own Fracture in the DVD, I'm not sure he does though). Switcheroo uses a gimmick not seen by anyone.
Of all of these, you might like Flash Deck Switch 2.0 the best, since it is tabled.Just to warn you though, it takes a lot of practice, and is not usable everywhere. In fact, I have had mine for well over a year I think, and I have never even used it in performance. At the same time, I don't do gambling routines, of too much tabled card work, and when I do I probably wouldn't use it, unless I had a bit of space.