When writing pin yin, should you use a question mark in sentences using "ma"?
For correctness, question marks are always required.

Are they redundant? Perhaps. If so, they are probably also redundant in English. (eg. "how are you." is clearly intended as a question even without the question mark)

"Ma" turns statements into questions. Think about "ma?" as being similar to "correct?" in English. (eg. "you like coffee, correct?")