Why Cash out That Winner?
Can You Articulate a Reason?
When dummy comes down, declarer's first task is to count out the hand, by which I mean, look at what needs to be done to ensure the most productive use of his cards. This isn't to suggest that "what needs to be done" will necessarily be unambiguous, for on the contrary, I'd say that on the majority of hands, declarer will have to entertain a couple of lines of play, to be decided later perhaps, when the hand becomes clarified, perhaps never to be clarified until he must play out the ambiguity (e.g., a two-way finesse against a Q).
Now at that point declarer has one salient task to perform in no trump contracts, two in trump contracts. The task common to all contracts (when you're not in a laydown situation) is to develop winners: get ruffs in the short holding before drawing trump, knocking out Aces and Kings, etc. to establish second- or third-round winners, etc. And in trump contracts, obviously another task is to get out the trump -- but maybe not too soon.
Sometimes these two primary tasks will be in conflict, and indeed, I have a category for drawing and another for delaying the drawing of trump. Each hand must be decided on its own merits.
However -- and the above is mere preamble to what I really want to say -- you don't wanna cash out top winners that you've always got access to just because you can. That is, you don't want to without a good reason until you're on claim (which would include a claim while conceding a loser or two). There are several reasons why such a common practice harms declarers' productivity, but I can cover all cases with one word: Why? Why do you feel a need to cash a winner to which you've always got access before finishing off the task of developing winners to the extent possible? If you can answer that question with a good reason, then it's part of your primary task. And if not, you'd almost surely do well to leave those winners alone for the time being!
Here are a few adverse results: Declarer wipes out a valuable entry to the other hand; declarer throws a valuable card third hand (substantially squeezing himself); declarer wipes out a stopper, allowing the defense a trick when they get the lead. And other malfeasances that don't come to mind right now but are given in the forty-plus collection.
Anyway, for illustrations of how cashing winners too soon cost declarers a trick or two, go here for some illustrations taken from actual play.