Hokey, if the bores and pisons measure up ok against the specs, then there is no need to get new ones if all is in good condition. If you rebore the engine, you will need new pistons. If the pistons and bores are worn, and you re use them, you will have a really rattley engine.(piston slap against bores)
If you are doing a Hybrid 2T/3T, you will need pistons to match the cylinder head.