If you've been told you can just get rings there is some truth to this, but not recommended. I remember years ago (in the 60's) that was a mechanics way to make money and it was actually common practice back then. You used to be able to buy rings that were wider/thicker and would compensate for the extra bore space, but they would never last. I don't think you can even buy those anymore.
For most stock-type applications, if the engine is worn normally, the block is bored, both to remove wear marks, make the bore round again and perpendicular to the crank centerline, and then honed to fit the oversize pistons procurred; this process provides the proper clearance for the piston and a suitable surface finish for the rings to seat against. Different pistons and piston alloys require different clearances; also, different ring designs/materials require different honing techniques/surface finishes.
Your machine shop is knowledgeable about such matters but an educated customer is an asset, IME..*G*
If you have any more questions regarding engine building, post...also, purchasing an engine building book would be a great asset if you are contemplating this in the near future...