You're definitely going to get mixed opinions on this topic. Like Zincubus mentioned, probably about 90% feed in the enclosure, myself included.
Snakes have very impressive digestive systems which break down bone and fur. A little substrate will not hurt them. They would have to ingest and swallow a pretty large chunk for it to cause any health problems. And, as mentioned, laying a paper plate, paper towel, whatever, will help alleviate this concern, but isn't 100%. Sometimes they will drag the prey item off the plate or whatever and inevitably get a little substrate.