Based on conflicting posts here:
I can see them raising the weapon up to aim, like your iron-sights do in third-person. So it would appear that they use sighted fire, almost exclusively.
From what I have witnessed it is always hip fire.
it seems like they use both.
Additionally, a post here:
They can use both, same for settlers.
combined with a post here:
I think it just depends who you use. Nick valentine is a hip shooter, but piper seems to be able to scope, who are you using?
Note: I realize these are all forum posts, but from all 3 places, the consensus appears to be that they both aim through the sights and fire from the hip.