In a straight wall case like the 454 Casull you should hardly ever if ever have to trim cases. I would not trim them unless their length was over the maximum listed length (unlikely to happen).
I don't know what kind of trimmer you're using. I know that with my Forster trimmer I would be able to trim 454 Casull cases as short as I wanted to.
If you're loading for a single shot handgun, you don't need to crimp the bullets; for a revolver you do. The "indentation" in the bullet is called the cannelure. If you need to crimp, the bullet needs to be seated so that the mouth of the case comes to about the middle (even a little deeper) of the cannelure. It doesn't matter if the case mouth comes to a slightly different area on the cannelure (a result of slightly different case lengths). On a cartridge the size of a 454 Casull, having a bullet seated a little more deeply than the manual calls for, probably isn't going to cause a pressure problem especially if you start your loads on the low side of what the manual calls for. If the bullet isn't seated as deeply as the manual calls for, the pressure would, if anything, be lower so that is not a problem. In a small capacity cartridge like the 25 Auto or 9 mm Luger, bullet seating depth would be much more critical.
