I like to crimp as a seperate operation from bullet seating. That way the crimp is consistant. I highly reccomend the "Lee Factory Crimp" die for this operation. If you have a good caliper, you can measure the crimp at the case mouth. The crimp can indeed have an effect on accuracy. Some powders require more crimp than others, depending on burn rate of the powder. Revolvers require a firm roll crimp, to prevent the bullet from moving forward on recoil and binding up the cylinder. (Under heavy recoil)
Savage