Ideally you would fit oversized pistons to a freshly bored block, to acheive piston to bore clearances that are as tight as they were when the engine was new....that way you will give the engine plenty of life. (provided the block isn't weakened by the rebore too much).

sometimes when you take an engine apart there isn't much wear and you can just deglaze and fit new rings and that alone will freshen it up a touch.