The arms only have to be "parallel" only if everyone of the opponents you anticipate encountering has shoulders of same height. If you do a cursory research you will see Yip man's don't all use "parallel" arm dummies. The dummy is only an abstraction of a human and not even an ideal abstraction.
I am assuming that when you say "parallel" you mean equal-height arms. It's been sometime since I have done geometry but my understanding of parallel is of 2 lines equidistant at any 2 points along the length.