I had a 0550 Delta flight out of IAD on Sun Feb 8. No line to check in at kiosk, and I was 2nd in line in the Precheck line a few minutes before 0430. It opened a few minutes after 0430, at which point there were maybe 3-4 people behind me.
So, yeah, 0430 would be plenty early; 0500 should be ok but I'm not sure I'd push it that far.