I would rather not creep. With good grass a good cow should be able to wean a 500+ pound calf without any crutches. Spending another $50 a head to get another 50 pounds of weight per calf generally doesn't pay (if you have low grain prices and high beef prices it might). That said, if you are in a drought or are pushing your stocking rate too high, or you are a fall calver and the winter is hard I would rather creep feed than see hungry cows and wean 400 lb calves. I know this didn't help much; but it all depends on your situation and what you are trying to do.
I use my creep for the calves when they are about 1 and a half months old. I use calf starter with decox as we have had coccidiosis before and this eliminates it, also the moms don't have to work so hard when I need to breed them back; but when they go out to pasture they get nothing but grass and minerals..