Consistency is the key, if you can consistently find the same anchor every time you will be accurate. Try practicing with a hood or cold weather clothes and see if you have trouble finding the same anchor quickly. If it's fine, leave it alone.
That being said, being able to put your thumb behind your neck tells me you probably have a very posterior anchor or your DL may be too long. For some people it works just fine, but for many it creates a "floating anchor" or one that is difficult to become consistent.
Another problem with a DL too long is it is more difficult to execute a proper release and follow through. Shortening the DL often makes your form more stable and easier to break the shot with back tension on the release. See where your nock is positioned while at full draw, a good point is vertically in line with your eye. With most bows' string angles that should put the string at the tip of your nose for an additional anchor point, and your knuckles at your jaw line for another anchor point which is more consistent and accessible in cold weather clothing. If the nock is behind your eye that's a good sign that DL is too long and more unstable.
