yep bbforks is right about the rental yards, they only raise thier booms and masts to get attention to potential customers. Thats the only reason they keep them raised up like that, apparently in that case money is more important than safety.

As for the idea of leaving the forks elevated at all?
No it will not hurt the lift but also it is not safe.

It is always a good practice to leave them on the ground and even tilted forward a bit to keep the tips down flush with the ground. But even doing so it still doesnt prevent slips trips and falls. You can always add an additional attention getter by putting orange safety cones on or around the forks if the lift is going to be sitting in one place for long. Or assign an area for parking the lifts that is roped or chained off with safety chain or something like that if you have room for it.
Anything to help prevent accidents.

Just my thoughts on this.