i had one with my first one and ended up with a 4th degree tear. sometimes, it helps but sometimes it just leads you to a tear. unfortunately, there's no way of predicting what's going to happen. i just had my second one recently and an episiotomy was out of the question due to the tear i had gotten with my first one. it was either vaginal delivery or a c-section. luckily, i was able to deliver with the help of a vacuum suction and ended up with a 2nd degree tear. that was something i could live with. it was so much better than my first experience. the second time around is usually much easier.
i don't know how far along you are but i would highly recommend doing prenatal yoga and kegels. good luck!