Read my original comment. It would very much depend on context, and, as I said, I'd be more inclined to do it in the false case.
It's funny, you still seem to think there is a "right and wrong" here, and you can't see that coding style is just like writing a poem - each individual will do things a little differently.
What context does it depend on? Where is it reasonable to say (x == true) instead of x?
Edit: your original post said "If I were writing something, and I really wanted to make it clear that I was testing a boolean to be true, I might write that." What does this even mean? Simply writing x isn't really really clear, so you write (x == true) to make it extra clear? if (x) is testing boolean to be true and it's crystal clear. What else can it mean?
I personally think it depends on the complexity of the current function, and the complexity of the variable x. i.e. if x has been read and written to a bunch of times in the function already, it might add clarity to be very clear that you mean "if x is false".
The clearest way I can think of to write that, that's hopefully less prone to misinterpretation is if(x == false) rather than if(!x)
Like I said, these are just my personal opinion and an expression of how I code (and likely things that I find difficult or often misread when reading the code of others)
It's funny, you still seem to think there is a "right and wrong" here, and you can't see that coding style is just like writing a poem - each individual will do things a little differently.