Greg,
> Only if your sample is random and independent. The existing mechanism tries
> fairly hard to ensure that every record has an equal chance of being selected.
> If you read the entire block and not appropriate samples then you'll introduce
> systematic sampling errors. For example, if you read an entire block you'll be
> biasing towards smaller records.
Did you read any of the papers on block-based sampling? These sorts of
issues are specifically addressed in the algorithms.
--Josh