MY daughter is going to college and I have been looking for a safe to store it. Most safes are 12.6 inches wide and her mac pro is slightly less than 13 inches, is it OK to store at a slight tilt?
Yes - the only problems with storing the Mac Pro are exceeding the temperature ranges or very hard drops. It's an amazingly solid machine, but at some point anything will break if dropped or shocked hard enough. Also, the Mac Pro is 9.9 inches tall and 6 inches in diameter, so you might fit it in quite nicely in a 12 inch safe since the longest dimension of the unit is 11.9 inches.
You might also look at the Apple lock for the Mac Pro and leave it anchored with a robust cable to a secure anchor.