I usually go by the rule of thumb:
- If you're adding to the profile and generally staying within the spirit of previous edits, but have more to share about the same - then in general, adding to is fine
- If you're proposing significant changes, removing pieces, tearing down and rebuilding, or hijacking their profile - then ask first, even if you know you're 100% right. Especially if it's connected to some non-open profiles where you can't see all the details, or could be connected to living persons. It's good to be cautious and polite and tread lightly when you might be bringing bad news, especially if someone has tried hard to research a profile and you're the one to let them know that either a mistake was made, or the data they've copied from has been proven incorrect. Either way, try to break it to them nicely.