Yeah most of it is genaral wear, but maby there is a big burst or a crack.... I would always take the pistons out when you have your block open. Maybe you rebuild your engine for loads of money and then somethings goes wrong because of a crack of to much wear on your pistons..

Also you can check the lines in the block for the oil to come round the pistons to lubricate it (don't know how you call in it in english :'()