You're actually probably better off not crimping your .243. If you're sizing die sizes your neck properly (which guarantee proper tension to hold the bullet) and true to the bore then your rifle will likely be more accurate than if you crimped your cartridges.

Like Vapodog said, (roll) crimping is critical on hard kicking revolvers shooting heavy bullets because the recoil and inertia are sufficient for the gun to literally act like a kinetic bullet puller. Guns with tubular magazines need to have their bullets crimped for obvious reasons.

I think that he left a couple reasons to crimp off his list though. If it were me I'd also crimp heavy bullets in hard kicking hunting rifles (for the same reason one would crimp a revolver bullet). I'd also crimp any round for use in a semi-auto firearm with whatever crimp was appropriate to that cartridge to prevent a bullet from being pushed back into the case during feeding, thus creating a dangerous high-pressure condition when fired.

But except for these reasons, neck tension is sufficient to provide an adequate grip on the bullet.

Mike