For optimal sound quality I would run all digtal volume controls at full bore. It's very doubtful that they will clip, though you may experience minimal distortion if there's some cheap parts in there. If you do notice distortion, dial it back a bit. If you don't, then just keep it full blast and turn it down at the hardware level (the dial on your speakers.)
The reason I say this is because the most god-awful part about desktop speakers is the signal to noise ratio. If you're running onboard sound especially, you're probably going to get some line hum from inadequate shielding at the motherboard. When your speakers pick this up in the line, they will amplify it. This noise is going to be present in the output at the same volume regardless of your digital output level, so it's advantageous to try to 'drown out' this noise.
Your desktop speakers also probably have a cheap amp in them, so you want to push this amp as little as possible. It's easier to boost line level sound than output level sound. Keeping the initial input high volume and the amplification low is advantageous all around when it comes to sound quality.
A nice sound card will typically have a high signal to noise ratio, and if you're using one then do whatever works best for you. If you're not running a nice card, I would recommend boosting digital volume controls for optimal sound quality.