Creep feeding doesn't cause them to have reduced milk, it is the fat. If you are creep feeding a calf that is on a good milking cow, and they get too fat, then it will mess with udder development and cause reduced milk production as a mature cow. I spent an entire semester in a lactation course, it was truly interesting!
Moral of the story, I would adjust your creep based on the year, cow condition and calf growth. I would say for the most part, depending upon the operation/operator, creep feed is not a high return on investment. Then again, if its a dry year the cows could use the assistance. Again, this concept relies entirely on what your operations goals/objectives are.