we don't opt out of all either, but there are good, considered reasons for the ones we do. I do have the 20w u/s b/c I had a cousin who was born w/ a complete diaphragmatic hernia (meaning it wasn't there). it was devastating for our family, and my grandmother said she thought her first baby died of the same thing (although no one knows for sure). So, I really ask only one question at that u/s "do you see a diaphragm?"
I agree w/ the mamas who said "only if it's going to change the management of the pregnancy." If our babe had a diaphragmatic hernia, we'd have the option of looking into medical interventions to try to correct the situation in utero. I don't however, do gestational diabetes, or GBS, instead opting to manage for risk factors (of which I have few). I have routine bloodwork since I'm prone to low iron.
a tip for the Rh- crowd. Get DP/DH to go donate blood...if he's negative, too, you won't need the shot! That's what we're doing. I figure that before I get poked this time, he'll have to get poked too!
there's my 2 cents...