I tend to shoot the image as framed in the viewfinder of the camera I choose. If 6x6, I tend to frame an image that looks good in the viewfinder.
Sometimes I break out of that mode and intentionally shoot an image that I know I'll crop differently, but that's more rare, and usually because I don't have the lens handy to properly frame the shot, and it is fleeting. So in that case, I might still crop to a square, or not, just depends on what I see in the resulting image.
But usually, when I'm looking through a square viewfinder, the image just intuitively gets selected and shot based on that frame, and tends to stay that way in the print.