Thinking about this for a minute, my sense is that there is no good reason to re-use pistons. Pistons and plugs should be considered throw-a-way items. After going to all the work to take the engine apart, clean it up, buy new gaskets and seals, why re-use the single wear part that is most critical to the engine running? Granted, if they measure within tolerance and look good, there is no reason that they could not be put back in. But if it comes to the question of blasting, they must need some significant cleaning. Since aftermarket pistons cost about $35 each - compared to the other costs in rehabbing a sled, they are worth the investment.