Is it safe to stay in a casa particular?
Casas particulares are very safe. Safety standards and the privacy of casas in Cuba are the same as those of a hostel or B&B in any other country.
The rooms can be closed properly in all casas. And the casas themselves too. In addition, most casas have a room safe.
The owner of the casa particular will also always ask you for your passport. And make a copy of it for the guestbook. That must be submitted to the immigration service on a daily basis.
If you are not asked for your passport and you do not have to enter any information in the guestbook, chances are that you are in an illegal casa. This procedure does not only apply to casas, but also to the resorts.