I usually lap a bit on general principle when building a gun. Most don't need it. The builders are pretty diligent about putting out good barrels, and it's darned hard to find a bad one from any builder. Shoot them a few hundred times, and they will generally smooth out.
The only gun I ever built that really needed lapping was a Judge Resley barrel that he had made probably 50 years ago. The bore was great, but the grooves were rough as a cob.
I'd put 500 rounds out of it, and it was still cutting patches. I impregnated some patches with valve lapping compound, and fired ten shots with around 10-15 gr, powder. Took care of the problem, and still shoots great today.