It was accurate for me two days before AF was due. The trouble with testing too early is its heart breaking if it turns out to be a chemical pregnancy. I held my breath for those two days! The longer you can leave it, the better and more accurate the test will be.
Is there any point testing 6 days before your period's due? I did today, actually 5 days before AF is due, having had some spotting the last 2 days, which I thought might be implantation. It was negative, but I don't know whether that's meaningful. (I know I should just wait a week to find out, but another week is SUCH a long time).