Dan, I'm unsure what you meant by:
For safety's sake lower current is better
I don't understand how driving the Arduino with, say, a 4A supply is less safe than driving it with 550mA.
I would go further, and say a 4A supply is more safe than a 550mA, as a 4A supply is running well within spec. and so should be unstressed.
If the Arduino tries to draw more than 500mA, and it is a normal Arduino, it's onboard polyfuse would (temporarily) blow, protecting the whole system.
Are you thinking of something else?
GB-)