Another good reason to always measure and trim is if you are seating and crimping bullets with a cannelure. This always insures that your dies contact the case mouth in the same position and it actually crimps into the cannelure. If your lengths are all over the place, you will not get consistent crimping and possibly change pressures
If you want to measure your chamber and see what it will actually allow for the max case length you can use a product called cerrosafe which is a low temp casting alloy. You can easily melt it and pour it into your chamber and let it harden. When you drive it out, you can actually use it to measure your actual chamber dimensions and observe the throat conditions. Lots of good info can be obtained for more advanced use.
Not suggesting it is necessary, however.