When riding with a single pannier, does it matter which side it's on? I read somewhere that when riding with a single pannier, the pannier should be mounted on the left side (traffic side in the US). However, the article didn't give a reason behind that. It's been quite some time and I can't find the article again for reference. 

I initially thought that it was so that the reflectors on the panniers wouldn't mislead motorists as to where on the road I, the cyclist, was positioned. However, after noticing this evening that a pannier mounted on the traffic side of the bike blocked the visibility of my girlfriend's seat post mounted light, I began to question this.

Is there any reason to ride with the pannier on one side or the other? Does it matter it all?
I wasn't aware of a rule for this, but I would hang a single pannier on left because it's the non-drive side of the bike. I doubt it matters though.

When I was buying a new rear wheel a few months back, I found the spoke patterns in some rear wheels are different on one side to the other, so I don't know if this would make a difference.

(I'd have added this as a comment, but it informs me I'd need 50 rep for that, hence the "answer".)