I have tried both sides for one pannier. Up until recently I used the left side (or road side). But because I am short and have a small frame the pannier blocked my view from my rear view mirror. I use drop bars with bar end mirror on the road side. So I switched to the right side and have found no issues at all, except that I can now see clearly out of my rear view mirror. FWIW I don't believe you are any less visible with the bag on the right side as compared to the left side.