I would think that saying to let it roll off the ramps is asking for trouble. On my ramps they have a shallow dip for the tires to sit in. Having the door open is a good way to get the door caught, injured or killed. If you are able to just let it roll off sooner or later someone is going to be taking the blocks out from behind the wheels and have the car roll with nobody in it. I see no reason for ramps to not be as safe most other forms of raising a vehicle up. A lot of people have been hurt just because they didn't use the jackstands correctly. For that matter some cars have come of lifts because the supports weren't put in the correct place and the lift area collapsed.