The brass does undergo stretching so as to exactly match your chamber when you fire it the first time. It's called fire-forming, and it's pretty normal. In an Ackley Improved cartridge, you put the unimproved cartridge from the factory in the chamber and fire it, and it stretches out to the "improved" dimensions.
Long as you neck-size the brass after that first stretching, no harm done. The brass will then fit yr. chamber perfectly with no further stretching.
If you get premium brass in the first place, it'll be a little larger for one thing--very tight fit in tight chambers, but a better fit in all the others, but also it'll be more uniform in weight, interior space, drilled not punched flash holes, etc, etc.
And that's the whole idea behind precision reloading: To make each cartridge as close to exactly like all the others as is humanly possible, given the limitations of our skill, knowledge, and equipment.
God Bless America