Let's say i take a pair of diodes and cut a usb cable and add the diodes to the Vcc and ground, and then clip stuff to the diodes to be powered by the 5v (or 4.5v, whatever it will be after the diodes)
Is there any reason I shouldn't do this, if i be sure the wires don't short? any risk to the pc or other devices on usb bus if i plug it in backwards?
You can do this, but I would only do it for very small loads. Without negotiating, any load above 100mA theoretically risks the USB controller shutting it off, or even cutting power to all the USB ports. (In practice, up to 500mA is unlikely to be a problem. But more can be.)
Unless you have a very specific
need for a device to be powered from the computer power supply, which it doesn’t sound like you do, I would use a separate power supply like a (high quality) USB charger or even better, a lab power supply.