Early u/s sac or yolk etc measuring 5w1d when I'm 5w5d! I know exact dates of conception as I used opk's and did charting. What's the margin of error?

Not "that" accurate. First-trimester measurement of the fetal pole/crl is accurate to +/- 3-4 days. It is not intended or presented as exact. Yolk sac and gs measurements are much less good; why were you scanned before the 6th week? What you've given is roughly within the limits of accuracy. Side note: charting and opk do not guarantee day of conception, they are also quite plus/minus. Good wishes.