I tried to deposit from poloniex to coins.ph a site that convert bitcoin to cash, I noticed that unlike other exchanges, coins.ph allow the coin sent from poloniex to be spendable after one confirmation. That was my first time experience to have bitcoin spendable with just 1 confirmation. In a standard basis it needs 3 confirmation for the transaction to make your bitcoin spendable in that site.
I suppose it's a little different if you are changing money to another form of money. In your case bitcoin to cash. Then you probably have to wait 1 or more confirmations. If there is a fork or you put some effort and created a duplicate transaction, their money is gone.
But if you are selling goods or services online I think zero confirmations is worth the risk. Mostly the amount of bitcoin is small, and the product is probably only shipped or used a few hours later, so there still is time to cancel it.