How long does it take the block to be broadcast into the network? If too short, bad guys could recognize the proof of work. If too long, other clients could "steal" our block with theirs.
If I understand you question, you are asking if some node to which miner broadcast its final proof of work cannot hijack it and use for yourself? It is not possible because 'proof of work' is unique to miner who found it (there is encoded reward for miner inside; found block is useless for other miners).