Depends completely on who you talk to. Very heated arguements on ar15.com about this.
Personally I don't for my AR and have had no issues. There always seems enough neck tension to hold the bullet. If you push the bullet as hard as you can against the reloading bench and if it doesn't push in, you don't need a crimp.
If you use a taper crimp, each round will need to be exactly the same length or ou will buckle the longer cases. This is fine if you trim each case each time. Personally, I only trim if they get to the MCL.