My m/w (who got here 15 mins. after the birth) waited until the cord had stopped pulsing and placenta was delivered to take blood with a syringe from the cord... babe is AB+ like daddy (I'm O-) and she gave us a scrip to have filled for the rhogam, which she was going to come and administer at the 3-day check-up, but we decided against it, since we don't plan on having any more children, and as this would be my first possible sensitization anyway, and birth and stage 3 were completely trauma free and physiological, even if I were to get pregnant again, chances are very slim that there would be any risks. There simply was no reason to believe that any sensitization would have occured.
I can also always go get a blood test, if I need to know for certain. I didn't want to risk the shot and all its nasty ingredients while breastfeeding.