Is it more to do with attitude or financial upbringing? Or both?
Growing up, my dad was a taxi driver and my mum did various low-paid jobs, but overall I'd say lower middle-class. Although I sometimes think adding the 'lower' bit sounds like I'm trying to justify something.
I always think of that old adage: 'If you know you're working class, you're working class. If you know you're upper class, you're upper class. If you have to sit around trying to work it out, you're middle class.'