If you do a Flip 0 and you don't poll the system for events then you will hog the cpu (core). Even if you do wait for events, you will still hog most of the cpu time.
Trying to draw an object off-screen takes some cpu and gpu time - the cpu has to send instructions to the graphics card about what to try to draw, and the graphics card has to think about whether any of the pixels are within the viewport. Of course it will clip the graphics since there is no offscreen areas in the backbuffer, but it still has to think about each pixel in the image to see if it's visible. So you do save at least some time if you can void even trying to draw such objects. But it's a matter of how much time you save, and for just drawing a bunch of 2d quads it isn't going to be that significant.
Trying to draw an object off-screen takes some cpu and gpu time - the cpu has to send instructions to the graphics card about what to try to draw, and the graphics card has to think about whether any of the pixels are within the viewport. Of course it will clip the graphics since there is no offscreen areas in the backbuffer, but it still has to think about each pixel in the image to see if it's visible. So you do save at least some time if you can void even trying to draw such objects. But it's a matter of how much time you save, and for just drawing a bunch of 2d quads it isn't going to be that significant.