Along with Nick's excellent suggestion, I just wanted to offer another thought. I have one client who really enjoys having the odd bar go all the way across, but having all the rest in a wrapper.
After looking at the Genesis HTML output, I realized that this is quite easy to do with a tiny bit of CSS, so now I do that routinely. I don't define the width of the outer wrapper at all, so it just goes all the way across.
Then for each subsequent DIV inside the wrapper I have 2 nice choices... I can either define its width so that it seems to be in a wrapper, or leave it alone so it goes all the way across.
But wait, there's more... what if you need, say, a graphic background image for a section that goes all the way across, but you still want its content to be in a "wrapper"? That's easy, too. You just use the .wrap class that's inside each section - define its width for that DIV, and you can have your cake and eat it, too.
You don't even need to write any function code for this method, just CSS.