It depends. The pistons in these little engines are so light weight, they really don't stretch rods like you see in a conventional gasoline engine. However, the bushings do wear, and cannot be purchased separately. I've rebuilt many with only piston and sleeve, but if the rod bushings are worn, I will replace it as well. On OS engines with the floating wrist pin, I replace it, but generally leave wrist pins alone unless there's a reason to change it out.
As for break-in, I typically do about 100 times the engine displacement. So for a .15 (2.5cc), that'd be 250cc of fuel, or about 3 tanks (most 1/10 tanks are ~75cc)