IIRC military spec ammunition in self loading actions is crimped, to prevent bullet setback while rapidly feeding ammo into chambers in combat.
For target purposes, a lot don't crimp, and it's acknowledged it's a complication in sorting out a good recipe for a load. However, military ammo is 2MOA specification, made on government owned equipment run by a contractor paid to do to spec and tested. They get the results. As said above, if you don't have a problem with bullet set back, then it's not a problem. Another reason for crimping, or at least having some neck tension, is that if the round is relatively loose, then as it chambers it may also be knocked off it's coaxial center. If the bullet tip is more than .002 off axis, then it loads and jumps into the rifling off axis, and spirals down the barrel to be projected at any position of the clock it arrives at. That causes dispersion, ie, opens up your group. It's considered acceptable if it's kept to 2MOA in a combat weapon, as the point of impact is an 18" center of mass target, or, 18MOA at 100 yards, close to the most common range at impact. Combat rifles are commonly still in use with 6MOA dispersion and some noted to 8MOA before a new barrel is installed, with well over 50k rounds thru it. Target shooters are known to replace a barrel at 2-4000 roundsbecause the group opened up .25MOA - knocking them out of the top ten. It would still be a great barrel for almost all other purposes, you could say it was broken in. You'd be money ahead to build a deer rifle with that used barrel.
Target shooters have much more precise demands and invented the coaxial runout check to reduce opening up groups. If little to no crimp is working for you, then checking runout is the next step in improving accuracy. If rough use, combat accuracy, lack of setback and uncompressed loads are important, you crimp. Considering how feed ramps are gouged into barrel extensions on AR15's, not crimping is a reloaders choice, but it was spec for ammo intended to be used and even cross level issued to other soldiers using different weapons of the same cartridge. It also goes to milspec bullets often have a cannelure to set that crimp in, thereby keeping it from bulging out and jamming while loading. That's been an issue with some reloaders using different bullets in alternative cartridges where that bullet was invented for bolt action use, no cannelure. Crimping with too much pressure that causes a bulge will also cause jamming. Using a bullet with a cannelure meant for a specific cartridge goes to getting the OAL set for it, not some incremental MOA improvement that other finesse may provide. Sometimes the quick and easy reloading gambit gets more attention than it deserves.
If someone is running plinking ammo for busting dirt berms in a semi auto I would at least Lee crimp it to reduce set back and compressing loads. The worst case situation is a compressed high pressure round which exceeds the brass cartridges pressure rating.
