For me, it's more or less the middle out of laziness. I can just fold the fabric to find the center, and then stick a pin there, and it's like easy starting point. For a corner, I'd have to get out a tape measure or something and that's an extra step to the process.

After that, find the nearest solid block to stitches to start with so I get something to easily anchor stuff to fairly early on. (A habit from one too many projects that overwhelmed with the confetti)