Playfield glass can shatter at any temperature if you rest it directly on concrete. Never do that, even if you are being careful.

I have a handful of games in my integral garage this season and recently tapped into a forced air duct in the ceiling to warm it up a little. I have always controlled the humidity in there at around 40-45% year round. So far the temperature in the garage has been holding at right around 50 degrees, even on the coldest days. I'm thinking this will be enough to keep the pins safe, but only time will tell for sure. I know that when I first entered the hobby, the general rule of thumb was that anything over 40 degrees was safe, and the biggest danger actually came from large and sudden temperature and humidity changes (like from opening/closing the garage door in winter).