I dare say everyone who writes at all regularly, even for casual purposes, knows that it’s vital to have access to a dictionary. And with so many of them now online for free, there’s really not much of an excuse not to use one.
But what about a style guide? Do you need to use one? And by “use,” I mean “have access to and perhaps own.” Isn’t that like a usage guide? No. A style guide is not a usage guide. Most of them contain some usage guidance, but that’s not the point of a style guide.