If replacing the HU with a screen then you can use the ignition line to directly power the screen just fine. Just make sure to use the included voltage regulator.
The screen shouldnt draw much more than an amp, and that is fine for the stock wiring.
It is always best though to use the ignition line as a trigger only. So you would connect the trigger line to a relay, and then the relay passes through a higher current (lower gauge) wire to actually power the device.
I would take the +12v from the PC as the trigger for the relay. Reason being when you have a SDC on your PSU, you want the screen on when the computer is on, not only when the car is on. So you turn off the car, and with me, I have 30seconds before the PC shuts down. With the relay tripped off of the ign, the screen would go instantly. But when tied to the PC, it goes when the PC goes.
I actually have it powered off of a constant source since I turn my car on and off so many times in a short span doing food deliveries.