There's a consensus now that it's more efficient for the network as a whole to propagate the blocks as soon as the PoW check is complete. go-ethereum and Parity already do that, we still propagate only after the blocks are fully validated and imported into the chain.

Relevant go-ethereum codes
https://github.com/ethereum/go-ethereum/blob/edc39aaedf526bfb7757a445c1f9dd42f45dc8d4/eth/handler.go#L694-L732
https://github.com/ethereum/go-ethereum/blob/edc39aaedf526bfb7757a445c1f9dd42f45dc8d4/eth/fetcher/fetcher.go#L635-L682

Note that the hashes of the new blocks area announced (with NewBlockHashes message) by go-ethereum only after sucessfull full validation.

Relevant Parity PR openethereum/parity-ethereum#9954