It's not so critical but here's how to ensure the car is, in fact level.
The spots on the garage floor where the car's tires stand need to be
level in relation to each other, only then will the car be level.
Mark the locations of the car's four tires on the garage floor. Get two
identical (in height) cans or blocks, a long straight edge and a bubble
The goal is to determine which of the four spots is the highest and thus
determine the shims needed to bring the other three spots (wheels) to
that same height. This absolutely levels the car front to back and side
Put a block onto say the LF and RF spots place the straight edge onto
the blocks and determine which is the higher of the two; put a shim
under the block so the straight edge shows level.
Then check the FR and RR and shim the lower. Then check the front to
back on each side and shim the lower, most likely the rear. Expect the
rear to be as much as 1' lower than the front so water can drain out of
After the respective heights of the shims are established drive the car
onto the shimmed spots; it will then be level.