I read the pattern and if it says start in the middle i do so, i am busy with a sampler that state to start bottom right corner- i let myself be guided by the pattern
While some of my projects suggest starting in the middle like Joan Elliots Summer Goddess, others state to start where one likes while others recommend counting from the middle and starting from the upper left. What to do? Do what works best for you, after all, it will be you spending the time and energy on the piece. Whatever is comfortable for you is best. :) Not the answer I usualy like to receive as I personally prefer "set" guidlelines sometimes.
I read somewhere that it was best for right-handed people to start from the upper left and I have pretty much stuck with that unless Im doing a smaller project. Then I may start in the middle as I wont be working on it for a long time.
I personally have always counted from the middle, but started stitching from the upper left hand corner as it prevents me from soiling finished stitches by having my hand resting on it. Especially long and detailed pieces that will take me a lot of time to complete. It works good for me, even if Joan Elliot recommends starting to stitch her pieces in the middle. ;P