I have a bench supply which I can use to power a breadboard at 5 V. Is it a bad idea to power the the Arduino by going directly to the 5 V pin? It sounds like both this method, and the USB are unregulated, so there would be no difference. Is it safe? Are there any caveats to this?
5V.This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
They wouldn't advise it because they provide on board regulation. By feeding it directly you're circumventing their own protection. If however, you're providing your own regulated supply there is no reason why you can't supply it directly.
In a perfect environment, USB should already be regulated. However given the cheap imported USB hubs and plug packs that are available on the market, I would certainly argue that a good bench power supply is no problems in this regards.