hello resourceful minds.  sorry to ask such a remedial question, but i'm
trying to figure this out and could use some assistance, and it's ok with
me if i seem like an idiot as long as my motor gets built right and i learn
something in the process.   
  
my motor is being rebuilt w/ heads from mark stephens, new bearings,
and mahle piston/cylinder sets.  i asked the mechanic about balancing, he
said it's up to me.  he offered the vague question that since all he was
doing (to affect balance) was putting new pistons in, why would i want
to balance things, as he's never had a problem from not doing so.  if i'm
not mistaken, things were (or should have been) originally balanced
(maybe?) w/original pistons, rods, crankshaft, flywheel(?), as a set.  so
if i'm using different pistons, wouldn't i want to have it balanced again?
am i missing the concept here?  is it something i even need to think
about?  please excuse my ignorance, illumination appreciated.  

thanks, matthew

ps - already read bob hoover's sermon, definitely worthwhile, but not
remedial/specific enough for my limited knowledge/experience.