I see several members who are quite active that edit profiles prior to proposing a merge, and another member told me she was told it was OK. I see this mostly where there are two children with the same name, or two spouses with the same name. But dates, places, etc may be different, and there are no definitive sources on either profile.
In most cases when editing the profiles, the members do not add any additional sources, just adjust dates to match or places to match. In some cases the change has mention of "why the change", such as the death date cannot be correct due to the birth of a child, etc.
In my merge proposal I will identify why I think the people are the same and why I think different dates or places should be documented. But, I have never actually edited the two profiles to make them match, mainly because that one piece of data might be the reason that they are not the same person.