Is there a tool to model rigging
If the suspension ropes aren't vertical, then you're making a (3D) trapezoid linkage, and changing the tension balance between the ropes (which will happen whenever you move the shelf contents around) will change both the horizontal position and the tilt of the suspended shelf.

Assuming you don't want the shelf contents spilling off, you should have each of the suspension ropes be vertical, which will make the shelf position and orientation independent of the load. However, @RedGrittyBrick is right: you'll need some sort of lateral support to keep the shelf from rocking back and forth; either cross-bracing, or horizontal bracing to nearby fixed points.