"Hello yourself" (???) is very strange to my ears, and I think it might sound rude if you said it in the wrong tone of voice. Just say "Hi" to your boss.

I wouldn't use "hello yourself", as it sounds very negative and rude to me (but I'm Dutch, not a native English speaker). I would say "hello" or maybe "good morning" when you first meet in the morning.

Ich würde auf ein "Hallo" meines Chefs auch im Deutschen nicht unbedingt mit "Hallo selber" antworten ...

"Hello yourself" would be a cheeky, perhaps flirty comment. The ending "... yourself" is cheeky: children use it to taunt each other ("Pig!" - "Pig yourself!").

Can imagine a young Sean Connery saying it with a raised eyebrow.

@CM2DD: Quite a nice picture given by you... Young Sean Connery saying "Hello yourself"... :-)

this expression is out of the series LOST and actually the way they used it was friendly, smiling at each other. but okay, I understand it's rather informal. Thanks!! :)