Is there a time that I prefer the use the left shift key over my right? Yes, when I'm typing a letter or character with my right hand. For instance, when I type a question mark, I'll use the left shift key because I'm using my pinky for the ?. However, if the key is sufficiently far away from the right shift key, I'll use my left.
Below is a complete list of characters that I type with each given finger:
Right shift: QWERTASDFGHZXCVB!@#$%^~
Left shift: YUIOP{}|JKL:"NM<>?&*()_+
I think it's because I actually type like I was taught in school. Who knows?