Believe it or not, I don't have a set way to write the letter X. So I've used a variety of those depending on various factors:
When I am marking a spot, etc: I normally use 7. But I have also used 8. 5 and 6 less so, but still can do depending on if my hand if going back up or down the page to mark something.
When I am writing in English: I normally use 7. But when I scrawl (joined up handwritting), I use the rounded X, so from top left to bottom left, top right to bottom right. Or if I'm not keen on writing clearly, it becomes 6.
When I am writing in Chinese: I normally use 8.
But going through this, I have found I use all of them barring 3 and 4 as this runs counter to the language I use and the direction of the writing I am required to perform (right to left, or left to right, or top to bottom). But in Chinese, where it could go from right to left, there are no writing strokes that end up going back up as displayed in 3 and 4. So these two I hardly use.
However, I am also an arty type of person. So when the need calls for it, and if I am working in a cramped space and can't move my hand around that much, I have used 3 and 4 before, but not really to write the letter X, but using those strokes to get the image complete.
I am right handed if that helps.