Horological Meandering

First those where you should not leave the chrono running.  Basically any chronograph which uses a "horizontal clutch", i.e. one where a supplemental wheel engages the chronograph seconds wheel should not be kept constantly running.  There are two reasons.  First the teeth of the supplemental wheel and the teeth of the chrono seconds wheel are triangular in shape and are very fine.  There is considerable wear which can occur if they are left engaged constantly.  Second, in order to prevent flutter in the movement of the chrono seconds hand, there is a spring which applies tension to the hand.  This tension lowers the amplitude of the watch when the chrono is engaged.  Thus, timekeeping is affected.  For both of these reasons the manufacturers of these types of chronographs advise against constant running of the chronograph.  Examples of these types of chronos:  Patek's manual wind chronos, Lange's chronos, Breguet's manual wind, Vacheron's manual wind etc.

Neither of these problems exist for chronographs which use a vertical clutch system.  There are no fine triangular teeth and no tension spring in the system.  Amplitude is hardly changed when the chrono is running.  So let the chronon run all you want.  Examples of such chronos:  all Blancpain chronos, Vacheron Overseas Chrono (which uses a Blancpain movement), Bulgari auto chrono (uses Blancpain movement), Patek 5960 and 5980, Rolex Daytona (post Zenith models) etc.

Jeff

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