If it's out of sight, probably fine. If you can see it, it may be fine or something may happen. Criminals won't know what is in the bags, will break the window to get at them, and will go through the bags elsewhere. (This has happened to me, in another country.) Very painful to return to your rental car and find broken window, especially when you have kids with you that you need to keep calm when all you want to do is cry. And then there is filling out policy report and dealing with getting back to the rental place with a car with a broken window.