Should I set my PC Audio to 5.1 or Stereo when using headphones?

[kaishlothon]

In case it matters I have a pair of Sennheiser Momentum 2.0 headphones with a Sound Blaster Audigy 5/Rx Soundcard.

 

[stdragon]

Set to stereo.

5.1 is for if you have quad speakers, a center channel speaker, and a sub. Proper stereo imaging is only obtained when the speakers are set a specific distance apart from each-other and the DSP is calculating the values of delay based on those distance values. With headphones, you won't get proper stereo imaging when set to 5.1