Look at the guidelines for each interview. They will give you what you need to know. As I recall, the HR/Tech interview specified "Business Attire" - which includes coat and tie. Unless told otherwise, you can't go wrong with that. My Tech interviewer wore shirt and tie while the HR interviewers were more casual.

Not sure how "onboarding" can go South - unless you make it go that way. Yeah, it was a LOT of redundant and seemingly superfluous forms and submittals. But everyone, to a person, was awesome and bent over backwards to help.