When should i use GPU if im rendering on cycles? Here is my setup:

OS: Windows 8.1 x64 
CPU: Intel Core i7-4702MQ @2.2 GHz
GPU: Nvidia GTX 750M

Almost 100% of my renders have been done on CPU because every time I try to render on GPU (knowing I'm using auto tile size) I always get a significantly slower render time. I've seen a lot of people recommending GPU for renders as it can take up a chunk of the time required to render. My question is: when is it recommended to use a GPU to render and when is it better to stick to CPU render?
Depends which device has better compute performance. Usually that's GPU in desktops and CPU in non-gaming laptops.

Also you can be baking simulations on CPU and render at the same time using GPU.

Latest 2.79 Blender builds offer rendering on both CPU and GPU at the same time.