I usually start from the middle to be sure that there will be enough space on every side of the fabric and then stitch up towards the upper left corner :)
Back in the day I always started top left. Then I had a couple of mishaps (one 16 count instead of 18 count, and another a compete miscalculation of fabric size for some reason). Now I nearly always start in the middle. At the moment I'm half way through a huge HAED so I started at the top left. Fingers crossed I got the fabric size right.