It is not common practice anymore. The American College of Obstetrics and Gynecology sets the practice guidelines for obstetricians and no longer recommends routine use of episiotomies. Here is a link to their policy http://www.acog.org/from_home/publications/press_releases... Here is an excerpt:
"Recent studies show that common indications for episiotomy were based on limited data. Additionally, there was a general underestimation of potential adverse consequences associated with the procedure, including extension to a third- or fourth-degree tear, anal sphincter dysfunction, and painful sex. Data suggest that women who have an episiotomy do not have significantly improved labor, delivery, and recovery compared with those who do not have one. Without sufficient data to develop evidence-based criteria for performing episiotomies, clinical judgment remains the best guide to determine when its use is warranted, according to ACOG."
The majority of serious (3rd and 4th degree- into the anus) tears occur as extensions of episiotomies. If you cut it is either too small and the mom will tear anyway, or it is too big and she didn't need the cut in the first place. Also, an episiotomy cuts through layers of skin and muscle, while most tears are superficial, skin only.
Also, when your skin tears it will tear along a naturally weak spot along a natural line. When you are cut the that doesn't happen.
I had second degree tears with both my babies. I didn't really feel the tearing and the healing was painless. I have witnessed some episiotomies and the healing and it has been much more difficult those without.
Definitely discuss this with your doctor beforehand. You will not be in a position to argue when the time comes and doctors generally do not ask your consent to perform the episiotomy. If your doctor tells you they will wait to see if you "need one" that should raise a red flag. Per the ACOG guidelines the only real indication is certain instances of fetal distress.
With proper help from your provider there is a good chance you will not tear at all. Slowing down your pushing when the head comes out to let the perineum stretch is a good way to avoid it. Good luck to you!