Of course it is mandatory in every transaction if we do it without escrow.
But I've also heard that 1 confirmation is not completely safe but it will be safer if the transaction has reached 4-5 confirmations. I forgot how confirmation was declared unsafe, maybe you still remember about that? I remember about the 51% attack, can they cancel a transaction that already has 1 confirmation?
It is not mandatory for every single transaction, it all depends on the amount of money in question. If you are selling a cup of coffee, you can safely accept a transaction with zero confirmations since the cost to attack the network to replace this transaction is significantly higher than the cost of one cup of coffee. The more valuable good you are selling, the more confirmation you should wait for to prevent being attacked this way. It also depends on the type of good you are selling, because it is pointless, for example, to wait for 100+ confirmations to transfer rights on your 5 million dollar house since the buyer of a house can't run away with it after the fake purchase occurred, which means you can always sue him in case he tries to deceive you.