it depends on the "do"

i tend to ask if there's anything we can do to help, as a lot of the time, it's not so much what someone brings but how they help. we're having DD's birthday in a couple of weeks, and apart from one friend that insists she bring something (we made her son's bday cake so she wants to help - but can't be here to help set up - so "stuff" it is), we don't need anyone to bring anything - but we do need help with set up or cooking in advance at home kwim?

it really comes down to the event, who is hosting, our relationship to them, what we have to do to get there (a lot of stuff we go to involves a lot of travel, so bringing food becomes a painful and impractical exercise). i don't think there is any rule to it though...