No interventions are ever 100% risk free. The risks associated with membrane stripping are accidental rupture of membranes and possible infection (because every time you introduce something into the vagina you are increasing the infection risk, which is why I'm not a fan of cervical checks before labor). It can also cause uncomfortable non-labor contractions which make it difficult for you to sleep and you need all the sleep you can get right now to be prepared for real labor. It can be painful to some women during the actual procedure and can cause bleeding.
Membrane stripping appears most effective when you are already 41 weeks pregnant. But at the same time, most women go into labor naturally around 41 weeks anyways even if nothing is done. I'd say that if you go into labor after 48 hours then it is not related to the membrane stripping.
My best friend wanted a natural birth and therefore didn't want to be induced. She had her membranes stripped once at 41 weeks (didn't work) and again 2 or 3 days later. She was cramping soon after the second membrane stripping and was in labor for real later that night, but then again she was almost 42 weeks at that point and perhaps she would have been in labor anyways without the membrane stripping.
Here's a study that compared the results of 22 different trials on membrane sweeping/stripping. It did show that routine membrane stripping did tend to prevent pregnancies from lasting longer than 41 or 42 weeks. But at the same time, 8 women would have to have their membranes stripped to prevent one medical induction.
Membrane sweeping for induction of labour.
AUTHORS' CONCLUSIONS:
Routine use of sweeping of membranes from 38 weeks of pregnancy onwards does not seem to produce clinically important benefits. When used as a means for induction of labour, the reduction in the use of more formal methods of induction needs to be balanced against women's discomfort and other adverse effects.
http://www.ncbi.nlm.nih.gov/pubmed/15674873
Personally, I would probably skip the membrane stripping unless I was close to 42 weeks and even then I'd do my castor oil regimen first since it is non-invasive.