I have tested the day AF was due 3 times, all negative. I have had 2 postives, and they were both well late. The first was when we got back from holiday, I think I was nearly 6 weeks pg at the time, and the other time I tested 5 days late and was pg too.
I can't afford to be a serial tester!! Its also easier for me cause I have a pretty good 28 days cycle, so if I'm late, there is more than likely a reason...... Why waste money when your body will tell you soon enough!

Fi