John, my training is to always make it parallel, not because of an average fighter or whatever, but because it is the first step in getting a student to focus their punches with intent. Part of training spirit.
Most often when students arms go flying high, low, right or left, its due to lack of discipline, focus and mindfulness of attack.
That being said, each fighter adapts to their own body type, their advantages and disadvantages when they start trying their gear against a resisting opponent, but they should have spirit and focus when them move to that stage, IMHO.
Guangzhou Pak Mei Kung Fu School, Sydney Australia,
Sifu Leung, Yuk Seng
Established 1989, Glebe Australia