Whenever one axle is on stand, I always make sure there are wedges and blocks to prevent the other axle from moving even if the ebrake is off. I've made mistake a few times and the car roll forward or backward, tip over the stand, and collapse (I was never under the car until I do a push test to see if it is stable).
Make sure your ramp has something (bricks, etc) around it to prevent the ramp from moving, and on top of the ramp make sure you have wedge and brick to block your wheel from moving. It is never perfectly safe under a car, but hopefully this will make it safer.