When to test?

I typically have a 25 day cycle. I'm fairly confident I ovulated this month on CD17 or CD18. Because implantation doesn't occur for 6-12 days after ovulation/conception.... But my period is due only ~7 days after conception.... Does that mean I shouldn't even bother to test the day before my period (that's the day I usually test)? It appears to me that if I was pregnant, I couldn't possibly have any hcg until after my period due because I wouldnt implant until after my period is due.