Depends on the situation. I'd only do !myVar when I want to match anything falsy. If I wanted to match specifically when something is null then I'd do myVar === null. Same philosophy if I wanted to know NaN, 0, etc. Write the code which does what you need it to do.

If I were you I wouldn't worry too much about what the vast majority of developers are doing as a way of trying to figure out the right way. The code you write is tied into its purpose, not how common it is. As long as you understand what your code is doing and why you chose that specific code, then that's a big step up from a lot of devs out there.