There is lots to say about safety, but I am going to write down only few points.
Not only, that it is dangerous to let the tied person to walk around room or even let her live the room, it can also be dangerous to do shibari when she stands. When she is not tied to any stable object so she could fall, it is risk. She can faint suddenly and fall down and either to break some bone or even hurt face or teeth. There can be more reasons. First - if your bottom is new to shibari, you don't know what can happen. Maybe she just have some kind of allergy or she is claustrophobic or she is not in good mood or psychical state or she just forgot to breath correctly, or maybe she got cramp in the leg... So many possibilities, which can ruin bottom's confidence to you. It looks like it would be more safe to do Shibari when the bottom would sit on ground or in bed.
Another point I want to prepare, is to be ready to talk with my bottom if she is new, before we do first shibari. To consult with her the aspect of safety and tell her what is expected from her and what is not expected. Also there is lots of questions which I should ask like her health status. She also must know, that any kind of tingling or numbness in fingers or muscles is signal of danger that must not be ignored.
One of the most important things is to continuously check her fingers if she can move them and wrist drop, if the wrist is not drop. That would be a signal to stop and release the tie because one of the nerve was pinched and could be damaged.