I like to go with a medium grade of ammo for checking the accuracy of a rifle. I'll use a bunch of different brands until I find what the rifle likes best. Every rifle is different.

BTW the "break in" period is not something you need to worry about unless you have a burr in your barrel or especially on the crown. You might knock down those burrs and you could call that a break in but for the most part the stuff you hear about loading, shooting, cleaning and repeat at specific intervals is just not needed. I've never seen a rifle that needed to be "broke in" other than ones with burrs. That's only happened maybe twice in all the rifles and pistols I've ever bought. And I've bought my share over the years.