Personally, I don't log questionable time ... everything is either PIC, or P2 when I am under instruction (flying with a qualified instructor). But I agree that the bottom line is that it is your logbook and you can essentially put whatever you want in it, as long as there is nothing potentially misleading.
BTW, the phrase "legal document" has no real meaning. Pilot logbook entries would likely meet the criteria for the 'business records' exception to the hearsay rule, which I guess is what englishal's friend is getting at.