I have always started at the top left, but my newest project I have started in the center. My daughter always starts in the center, and her work is beautiful; this new piece is the most complicated that I have ever attempted, so I decided to do what she does. I am only a few rows in and I have already had to go back and take out a couple of rows because I missed one.
I find starting in the middle makes it easier to center a pattern without error and less effort. But on my current project, a HAE, I did what they told me and started in a corner. I chose the bottom left as sewing upwards uses less thread than down. And easier to go with the numbers one way at least. Sadly I'm slightly off-center.
If I'm doing something rather small I'll likely start in the middle, but for any projects that are even the slightest bit big I will grid and start top left. Especially if its a pattern with lots of pages, I start on page one and do them in order.
For me it depends on the chart too. Some I start in the middle, others from top left. The one I am doing now I started from the middle and then went to the lower right corner :).