If you want to save as much battery power as possible, leaving it on integrated only will ensure that the power hungry discrete GPU doesn't fire up.

With GFXCardStatus, you can monitor your graphics card in Macs with separate and discrete graphics cards. If you have a discrete GPU on your Mac, you’ll appreciate the app’s ability to make gaming easier. That will cause the discrete GPU to be used all the time. For example, if you don’t have access to your Notification Center, we recommend starting there. If your goal is better performance regardless of power consumption, then the solution is really simple: quit gfxCardStatus, delete it, and disable automatic graphics switching in System Preferences > Energy Saver. A drop down menu provides you with three options: Integrated Only Discrete Only and Dynamic Switching, which is the system default that lets OS X decide which graphics card is required. The author of gfxCardStatus answered here. Sitting up in your menu bar, gfxCardStatus shows you at a glance which card is currently active, but more than that, it actually allows you to manually switch between the GPUs. That's where gfxCardStatus comes to the rescue. It's often tricky to know which card is currently in use as the switch between the two is seamless. Some newer MacBook Pros have been packing dual graphics cards for a while now, with the ability to dynamically switch between them - integrated for battery life and discrete for performance.

Gfxcardstatus not setting discrete only