So your saying long as the hash that is used is from a block and not a transaction is ok?
Not entirely. The block reward is 25 BTC. If I'm a miner and find that a block I just mined makes me lose a 1000 BTC bet it would possibly be in my best interest to withhold that block and let someone else mine a different block.
I feel relying on block hashes makes it too complicated*, and transaction hashes way too unreliable.
*for me, that is.