|There is no consensus on this question, even experts disagree vehemently. For them it is a fine point in the discussion of expression. Usually this question is asked by beginners seeking a mechanical means of interpreting written music. In this context it may be the wrong question, as it pertains to just one element of expression. Notation is imperfect and cannot tell you exactly how to play the music. To do this you need to be familiar with the musical tradition to which the music belongs. Record your playing, and based on listening to lots of music of the same type, or, even better, to a good recording of the particular tune, adjust your play to match. Because beginners usually rush, a good starting point is to try to play the grace notes on the beat.