Is it really necessary to put blocks under the car to raise the tires off the ground while the car is in garage for winter?

 

If you plan to not drive for over 2 months at a time, blocks under the axle ends and front lower control arms can help prevent flat spots on the tires.
If you plan to move the car every month or so and keep the tires inflated at 32 or above, no need to use blocks.