what is good practice around initializing variables
Initializing a global is appropriate if you know in advance that it should always have a given value at startup. The compiler will generate a globals image in ROM that will be copied into RAM before any of your code is executed.

Safe to assume compiler will initialze?

The compiler will initialize to 0, a global not specifically initialized by you. Let's take the case of your Boolean. '0' means 'false', right? So 'x' is pre-initialized to 'false'. But I, as the reader (or yourself, six months later) may not know whether you intended this to be initialized to 'false' (the default value) or forgot to initialize it. Specifying it to be 'false' may not have any different effect on the code, but it does inform me or your future self, that you meant it to be set 'false' at startup. Remember that your code communicates to people as well as to the compiler. Write it so you or I will not only be able to understand it, but won't be able to misunderstand it.
If you initialize it in setup(), the compiler generates code in setup() specifically to do that. If the setup() function needs to decide during startup, what the initial value of that variable should be, then this is the place to do it. Otherwise, the extra code just takes up space in Flash. It's not any more harmful than wasting those few extra bytes of code-space, but it doesn't accomplish anything more than letting the globals image do it.
Whatever you did or didn't do with 'x' at load time or in 'setup()', it will persist into 'loop()', so 'x's state isn't unknown when you get there. (If I misunderstood that part of your question, please edit it in your question.)