When is the best time to use my sourdough starter?
From my experience, using the starter when it is at it's peak activity will help your dough rise faster but not necessarily higher.  There are a lot of factors that contribute to how much dough expands when baked including, but by no means limited to: loaf shape, docking pattern, oven temperature, and oven humidity.  

Be aware that there can also be a point at which dough has risen too much and the structure and texture suffers.