Once you have your cases the same length you may find that you won't have to trim them for many firings. All the chambers I have measured from custom to factory have chamber necks that are quite a bit longer than the case. It can very long in factory chambers and somewhat shorter in custom chambers. Naturally if you have the chamber dimensions you can do easily determine the end of the chamber and trim accordingly.
Even with the chamber dimensions I like to measure to absolutely know when the case neck is going make contact with the end of the chamber. You can do this with a sinclair chamber length plug or if you have a lathe you can easily made your own.
Varmint Al's webpage has a discussion as well as chamber plug dimensions, thanks Al:
useing a single shot like that for hunting and once and a while plinking thats up to you. But, if you are long range shooting or want to put that bullet exactly where you want every time then you may want to take the time to remove as many variables as you can. Sise, trim, clean the brass every load, weigh every charge and seat to exact OAL every load.
That is the best about reloading you get what you put into it