As others have said, whatever feels comfortable and whatever works. Your anchor is your rear sight. Using your index finger lowers your rear sight vis-a-vis using your middle finger if you anchor them in the same place. If you shoot a light bow, using the index finger will extend your point on range. At any given distance, you won't need to aim quite as high with the index finger anchor as you would with the middle finger anchor, because the higher aim is built into the lower rear anchor with the index finger. The differences aren't great, and your brain will quickly accommodate either anchor. Good luck.