The crimp in a straight-sided case serves two purposes. One is to prevent bullet migration, either due to recoil (outward movement, as in revolvers), or magazine pressure (inward movement, as in tubular-magazine lever action rifles). My 458 Win Mag is a Ruger #1, so I have no concerns about migration.

The second purpose is to increase bullet pull, so as to ensure proper combustion. If insufficient pressure builds in the case, combustion will be incomplete. The shoulder of a bottle-necked case provides this function. In a straight-sided case the increased bullet pull caused by the crimp raises pressure long enough to aid combustion. This is easily observed with slow powders. With the same load, but with and without crimp, a considerable difference in muzzle velocity can be measured. With faster burning powders the effect is less obvious, but present.

The only accuracy deficit I have observed that may be attributed to the crimp involves an improper crimp. If the bullet seating die is adjusted to seat and crimp at the same time, shaving occurs. I have seen this with cast and jacketed bullets. This is detrimental to accuracy, markedly so. The crimp should be applied in a separate operation, either with a special-purpose die or by readjusting the seating die.