I'm in the process of converting an ATX PSU into a lab supply. It of course has a 5v output: I'm not exactly sure how precise that is. I was thinking of using it to supply Uno through the USB by hacking a cable. What actual voltage is ok through the USB?
I do have a nice regulator like this
which I could easily use to make sure the 5v is in fact 5v. Beauty of that regulator is that the input can be below or above the output, so I'm good no matter what. The regulator's 300mA is enough, since the whole point of the PSU is to be able to supply all other equipment - servos etc- without going through the Uno. Even with a few loaded IO pins at 40mA each, I think I'll be good.
Alternatively I could regulate the PSU 5v up to 7v - that's a bit easier since I can simply jam a wire into Uno's Vin. Of course that seems a bit Irish since the Uno will regulate it back down to 5v again..
As an aside, there are loads of Instructables on using an ATX PSU on the bench; I like this example