There is no harm in testing all of them (thats if your using your own testing kit). You're only scraping off less than half a percent of a pill, so proportionally your losing stuff all MDMA (if there is any in there). A single testing bottle will do 200 pills, but it is likely to go off before you can test that many. There is no harm in testing them all and you'll be much surer at the end of it.
Pills can vary greatly from batch to batch, but it is also surprising how much a pill can vary from one side to the other. But at a minimum I would test every third pill in a batch.
Good luck and happy testing...