I know a couple of LJ'ing goths who now have Google+. Thus I can see people with stated first names like "Persephone", "Ariadne" etc. The rule of thumb is that if someone says that's their name, that's what you call them. There are also some clearly pseudonymous accounts, e.g. last names like "jumpycat", "Purple" etc.

It's impolite to ask what their "real name" is. I've seen someone (call her X) get upset because someone pressed the point of finding out what X's mother used to call her (if that's a a "real name" in any meaningful sense). And it was pointless too, what could you do with that information?

There's lot's of inaccurate information there anyway, I have two "Jo"s in my friends list. On is male, one female. In both cases "Jo" is a contraction, in one case of a middle name. They also chose to be called "Jo".

I don't really see why Google+ needs to know or care. What people answer to is what people know to look for, so that's what should be shown.