I wouldn't say marking is mandatory, but it will definitely help a lot. And there will always be those DDs who completely ignore any targets you set anyway and attack whatever they feel like (too many parties I've tanked in have had that happen sadly).

Really though, the best advice to the OP is (and I learned this in FFXI), is 'find what works the best for you for your situation, and use it'. Some like to set number marks to show the order to attack enemies. Some just stick a single mark on the first target and work from there. Find what works for you and use it, and as long as it helps you and your party, then that's what's important.