You are correct, you can crimp in the seating die if you are using a bullet with a cannelure, if not you need to use the factory crimp. What are you shooting it in though? If it is a bolt gun, I wouldn't crimp at all unless I was trying to find some accuracy for something special. I have shot thousands of 223 rounds in bolt guns, and have never used a crimp, even with bullets that have a cannelure. I would only use crimping as a last resort if you are looking for something to help accuracy. In my experience it can help at times, but not enough to make it worthwhile for me to do it.
If however you are shooting it in a semi-auto, someone else might be of more help, as I have never shot any in a semi, and do not know if a crimp would be necessary.
In my personal opinion as well, if you did intend to crimp, use the factory crimp die, I feel you can get a more consistent crimp than with the seating die. And you don't have to worry about getting it right on the cannelure, you can seat to your preferred depth and crimp. Some people don't care for them, I like them, but use them sparingly. HTH, Billy