At our antenatal classes the midwife (who seemed to be very into natural births and interception only when absolutely necessary) said if they think you will tear a lot they will do an episiotomy because the danger is if you tear from your vagina down to your anus you can be in a lot of trouble. When they cut, they cut off the the side to prevent this. She also mentioned the importance of a slower birth (eg. stop pushing when they say to stop pushing!) in preventing tears or cuts.