OP your post really made me think because I use "no worries", "no problem", and "you're welcome", and with my friends "no problemo" for a bit of variety. When I think about I use no worries and no problem in work situations when someone thanks me for doing my job. I think that I'm saying 'no need to thank me, it wasn't a big deal'. However, if I have done something that I didn't have to do to help someone out such as hold a door open, or pick up something they have dropped I am more likely to say "you're welcome" because they are thanking me for 'extra' effort that I have gone to. In other words, what Another Sarah said.