Personally I don't think car covers are needed, especially in a garage. You need to let the car 'breath' with some air rotation, the cover will just block everything. Being in a garage it's protected from the elements, so not really needed in my view.
Those cars that are left to ‘breathe’ in barns don’t fare too well.
Assuming the roof doesn't leak and the floor doesn't flood, ventilation is required to keep something dry in an unheated building. A car in a ventilated barn will get damp, because, often there'll be a spell of very cold dry weather followed by milder damp weather. Damp air gets into the barn and the moisture condenses on the car which has been cooled below the dew point. You are relying on ventilation to take the moisture away again when the dew point rises.
Covering the car is a tradeoff between inevitably slowing this process by interfering with the ventilation and preventing dust, bird poop, etc. settling on the car.
There's no right or wrong answer. You just have to decide based on the environment what's best. Make sure the cover is of a breathable material, though.