When does a repetition stop counting?

The final repetitions of a difficult set rarely look quite as controlled as the first few. The movement may become slower, the range of motion slightly shorter, or you may begin using a little momentum to complete the repetition.

At what point do you decide that a repetition no longer counts?

For example, if you complete eight clean dumbbell rows and then use some body movement to finish the ninth and tenth, would you record ten repetitions or only the eight performed with consistent technique?

Does your standard change depending on the exercise? A slightly untidy curl or lateral raise may pose less risk than letting technique deteriorate during a squat or deadlift.

It would be interesting to know where members draw the line between an acceptable hard rep and one that shouldn't be recorded. Has being stricter about rep quality helped you measure progress more accurately?