This is something my wife's aunt, who is a nurse (but not in any maternity or pediatric field) advised us on. She said we should avoid giving onions to my wife (who doesn't like them usually anyway) while she's breastfeeding, because it can make the milk taste funny and thus make the baby fussy about eating it.
The main question is in the title. However, if it's a yes, to what extent is it true? Does onion powder or dehydrated onion have the same effect? Does fully cooked onion have the same effect? Does raw onion have the same effect?