Confirming PSU spec for steppers.

[Jonathan]

Is it better to match up the driver output with the motor rating exactly, or leave a bit of a margin?

In theory you should be safe to run any driver up to its rating, since that's the whole point of a rating, if you can trust it. In reality however it's always best to overrate the drivers if you can, since then they will run at a lower temperature which prolongs the life of the components.

Also intriguing, though how would you establish that? Would it be as straightforward as checking the voltage from each output separately?

Yes - the difficult bit is determining how much of a mismatch would pose a problem.