If you are having your soft tissues smashed when you lean your pelvis forward, then something ain't right with your saddle. Might be it's angle, might have something to do with your reach to the bars which can be several things, including the wrong size bike.
Might just be you have the wrong saddle for you. Too much cut out can be just as bad for some as too little cut out. Some people don't need any.
As for my back, I try to keep it straight. But I don't know that I even have a picture that shows me what my actual position of it is. It's just all in my head! <grin>