A case trimmer might not be necessary immediately or with certain cartridges that do not tend to "grow" as they are resized over & over-some are a lot worse than others. For example, the .220 Swift, .270 Win, and the .300 H&H Magnum stretch alot the more often they are reloaded. But IRCC, I never had to trim a .308 Norma Magnum case!

The requirement is that you have to keep the case necks short enough that when the cartridge is chambered, it is not long enough for the case mouth to get jammed into the forcing cone of the chamber. If this happens, the case can get too tight a grip on the bullet, thus driving chamber pressures up, sometimes up enough to cause a serious "accident"! Just be sure never to let your cases get longer than the specified MAX case length, and you will have no troubles.

(Isurvivedwithout a case trimmer for my first 6 years of reloading-I used myRCBS case mouth chamfering tool by hand to keep the cases short enough to be safe- very crude, but it worked! No, they sure weren't all the same length,looked bad, and would not have worked if crimped. Probably had a detrimental effect on accuracy too, but in those days I was too ignorant to notice! But I still don't crimp bottlenecked rifle cartridges.)