I've never heard of a string breaking or a piano being damaged (i.e. actively needing to be repaired) after being prepared. That said, your question is the specific reason why many institutions / organizations do not allow their pianos to be prepared: they are afraid of the possibility of damage.
This is similar to a violinist refusing to play col legno with their $20,000 bow. Often times they'll whip out a cheap "beater" bow that they don't care about. Similarly with pianos, many institutions have less-wonderful pianos that they won't mind if they get damaged.
Will using screws 1 time to prepare the piano hurt the strings? Not likely. But doing it repeatedly, in the same locations, for long periods of time? I think about the most damage one would see would be that the strings wear out faster. At around $1000 to replace, not everybody can afford new strings.
That said, it also depends on what you use. Objects like pieces of paper, plastic straws, and paper or wood are all less harmful to the strings as their material is softer. Any object whose material matches or exceeds the strings in hardness will damage the strings (this same principle is used in percussion playing).
With screws specifically I think it would have most to do with thickness and with how pronounced the grooves are. A machine screw will affect the piano differently than a wood screw. Whatever you use, I fail to see how a performer might get injured (unless they're playing a piece where they frequently move from playing outside to inside the piano, violently and loudly).
Many performers shy away from this type of music because:
- It takes a lot of time to prepare a piano
- It's hard to find an available piano
- They themselves are afraid of damaging the piano