You don't always have to crimp on the cannelure , yet it's is a good idea for a couple reasons , 1) it keeps your cartridge OL at a proper length and 2) it keeps from cutting into the jacket if you use a real heavy crimp .

If your using say Hodgdon's H110 or Win 296 ( same powder ) a heavy roll crimp is needed , yet with some other powders like 2400 or Unique only a mild crimp is needed , just enough to hold the bullet during recoil .

stimpy