When do you need to use the XSheet?
Xsheets (or exposure sheets!) are essential for hand-drawn animation (on paper) but with the magic of technology, I never use them anymore. They're for timing out and planning your animation to make sure you're animating the correct amount of footage and for recording which mouth shape goes on which frame if you're animating to dialogue. But since TVPaint (and pretty much any animation program) allows you to hear sound as you scrub through your animation and change the timing of your scenes super easily, it's not really necessary.
I actually just looked in TVP though and it looks like there's a window for it! It's in the Xsheet tab in the Layer Panel. I didn't realize it was there, haha. It looks like it's just a different way of looking at the timeline for the more traditional animators who might find the Xsheet a more familiar way of looking at an animation timeline.
So to answer your question, you can use the Xsheet anytime you feel you might need it. I can't quite figure out how to use the one in TVP though, so if anyone wants me to give it a better shot, send me an ask and I can try.