This is an interesting discussion and Larry Horowitz' comments are encouraging. Years ago it seems U.S.P.C. gave us a much better selection of standard poker decks from which to choose. I used to love the "Aristocrats," which handled well for me. All the cards today seem to have a different kind of finish from the old "linen." While they fan well today (once upon a time you needed fanning powder to get cards to fan the way they do now right out of the box and never seem to stop), they also (I think) warp, bend, and crimp excessively. Further, they never seem to "break in," i.e., soften up to a condition that is optimum for close-up sleights. Stage card-manipulators have their own issues with the cards available today for that purpose. My old friend Jeff Sheridan made that the topic of his article in a recent Genii issue, which coincidentally followed a long telephone conversation with me in which I was whining to him about the problem I was having getting back into card magic due to the decks not seeming to break in anymore -- he commiserated and offered professional insights on the problem which seems to have been on his mind anyway hence his article. I suggest you take a look, it ran several months ago -- Jeff is a great magician and knows what he is talking about.
Perhaps people just starting out with these cards are not affected as much since they never got used to what now seems the luxurious earlier packs. In any case, when a deck passes what is for you its optimum state you will generally want to start on a new one. Until they are completely worn out you should work with several decks in different states of wear, as others have pointed out, to maintain your flexibility. Again, today I am personally not sure what "wear" means anymore, but then the issue is not as pressing for me now as it once would have been.
In any event, you should expect to go through gazillions of decks. When this thread started you were on deck #3. Keep practicing and good luck!