A lot of times customers request 'ratings' on components used in assemblies. It can be a difficult question to answer because there are so many variables.
For instance, one manufacturer might specify a given capacitor as being able to withstand 5kV, with a thickness of 0.1 inches. Another manufacturer may specify the same voltage, but a thickness of 0.12 inches.
At VMI, we tend to be conservative when it comes to voltage ratings on capacitors, but it depends on a lot of things - for instance, current draw, operating temperature, and frequency, just to name a few.
One solution is to specify the volts/mil stress and let the customer decide if they're comfortable with that.
I always liked the story about how Orville and Wilbur Wright used to do it - they'd switch out a nut or bolt until it broke, then beef it up a little. They used trial and error to specify minimum limits.
How do you deal with component ratings? Do you assume the manufacturer includes a margin of error? Are you comfortable with pushing the limits on maximum ratings?