Do I need to fill the gap of a wide tv mount hole?
As long as the mount bracket is secured solidly to the TV, and not putting lots of pressure on one particular spot of the plastic chassis, no, you don't need spacers. 

I've had mounts that come with spacers but it really just depends on the design of the TV and mount. If you can notice the plastic flexing while you put the mount on, then you should probably consider spacers or a different style mount.