Necessary - no. All current browsers can run pre-ES6 JavaScript without transpilation; and it is possible to write whatever you need without ES6.
I'd recommend it though, even though that means you need to transpile your production code so it works in all browsers (eg. with Babel).
This is particularly true if you are learning JavaScript now - if that's what you mean by "start"? You might as well learn ES6 as the majority of browsers support it now and it is the ongoing standard. So you may as well learn how to write JS in ES6 style and not the older style.