The writing section matters....a little. I think that the writing section is nothing more than a safety valve to help prevent people who cannot communicate at all from an easy acceptance to medical school. I didn't study for the writing section at all and got a P. Not great, but it proves that I can at least communicate efficiently enough to be understood. I doubt that anything above the 20th percentile would draw a red flag at any school. However, it may make the difference between you and someone that scored very close to you. Still, time is better spent preparing for BS and PS. Only practice for it if you feel like you really need it. I don't concider myself to be a great writer and I have horrible doctor-like handwriting. There's nothing that I can do to make my scribble neater.
This sounds like a stupid method, and it probably is but here's how I approached the writing section: I wrote a quick outline to organize my thoughts at first. Then, I proceeded to write very short sentences. I used very few compound sentences to prevent punctuation errors and run-on sentences. I took the concervative route by thinking that errors would hurt me more than short sentences. Childish, I know.