If your source and amp are truly of high (or reasonable) quality, you do (a). Some poorer sources have issues at max volume, and some amps will clip some inputs that are too high in amplitude, so in practice it is a safer bet to set the volume to 80% or something else, as others have described.
Lowering volume on the source means reducing the SNR. The music becomes more noisy, relative to how loud the music is. Thus you want to avoid lowering the volume on the source as much as possible, unless this creates issues.
The exception to the rule is if you're using exceptionally sensitive headphones, generally some models of IEMs. Most amps, particularly those that aren't that expensive, use a dual potentiometer for analog volume control. That's the volume control knob. Because of the way those work in these circuits, at the bottom range of the volume control, the left and right channels may no longer be matched very closely in volume. If the source is near max volume, with particularly sensitive IEMs, you may need to turn the amp down really low, into this region where the left and right channels are annoyingly not quite the same volume. Also this means that it's easily possible to accidentally turn up the volume a lot and blast your ears off.
