In every facility I have been in we always flushed with just about 5cc of NS or about 3.33 cc's in each one since some nurses get lazy and only want to use 1, 10cc syringe

. Part of the logic behind that is we draw labs from the CVC's and most facilitys I have been in don't want you drawing PTT's PT's etc. from CVC's that have recently had heparin infused (although you can waste 10cc's of blood, pull out another 10cc's of blood for your CBC/BMB etc, then pull the blood for the PT/PTT after that and you should be okay, or at least that was the policy in most places I worked...your mileage may vary.
We didn't pull out the heparin until the line was occluded (which 99% of the time is because the last couple of shifts before never flushes the lines).