This is just my $.02, maybe some individual or some entity like your County Extension or the USDA has done more exacting studies. I just simply let them hang and bleed until the blood stops dripping out. I slit the throats, which is supposed to keep the heart beating reflexively and forces more of the blood out.
I've butchered chickens at a friend's house who wrings the necks, then stuffs them in a cone and chops off their heads. It seems that the birds he does that way have more blood oozing out on the cleaning table than the ones I do by slitting. The meat also seems pinker/redder on his birds than mine.
You can experiment, let them hang for a certain amount of time, or until they stop dripping, and then see how they look when you get them on the cleaning table. If there still seems to be more blood oozing there, try hanging them for longer.
I don't know about retained blood making the meat rubbery, I don't think that is a factor unless perhaps you didn't bleed them out at all. I think that the retained blood has more to do with appearance (blood-filled veins in the meat) and spoilage factors (more blood makes the meat spoil a bit faster).