I was in the DK panel at celebration and one of the questions asked by an audience member was about whether they are canon or not. Pablo talked around the question. You could tell he didn't want to simply say no. Pablo seems to enjoy not giving straight answers...(on a side note I was pleasantly surprised with the straight and honest answers Doug Chiang gave to questions in a different panel).
I have participated in discussions on the RPF forum where people spend a lot of time/money building a costume based on reference photos from the visual guides only to find out, once the bluray is released, that there have been some very large discrepancies between their references and what is actually shown in the movie. This happened just recently with R1.