I think the PC. I consider the volume knob on an amp to be a gain control, telling the amp to expect however much signal input voltage. Turn the PC up full volume, then turn the amp up until just barely below audible clipping / dynamic compression. Then turn the PC volume back down to a comfortable level. Thereafter, if you occasionally want to crank up the volume, you are still below overdriving your system, even with the PC volume at 100%.
There's no foul either way though. Just go with whatever sounds good to you. If you turn the PC volume up to 100% then control the volume with the amp gain, who's to fault you if it sounds good? Turning a knob is certainly more convenient than clicking a virtual slider.
It may depend on the computer or software you are using. Apparently when you turn down the volume in the digital domain, often it actually loses bits, depends on how it's been implemented. Now, in my experience this hasn't really been audible, but just be aware of possible sound degradation.
rojo: Any volume knob is gain control , so that doesn't really make the gain on the amp any different then the volume knob on the receiver.
Now the suggestion from rojo on how to set it so that it can never be over-driven is certainly a good idea! Especially if you have kids or roomates
Personally, I prefer to max out my computer volume, max out the gains on my amps, and control the volume with with a preamp.
But, I have broken that preference for convenience. My advice would be to do what is convenient and compare it to whatever you think might be better, then decide if any improvement is worth the extra hassle.
