70W is like 15amps... right?
Something like that. Sorry, I misread the W bit...
Yeah I use an old PC power supply as my bench supply which pumps out 35A at 5V. I also fitted someone else up with an old supply which is providing 12v for the LED light circuit in their 'train shed' (they have a lot of LEDs).
As I said, all of the ATX supplies I have tested give voltages very close to the 'intended voltage'.
My current supply (with no load) gives:
5.19 on the 5V and 5V standby (1.5A for the standby - used for most things)
3.31 on the 3.3V line
12.11 on the 12V line.
I have not had any issues with my supply or any others that I have converted 
Mowcius