Yep, its a bad idea to let your potential new boss know you live with your folks.
I'm sure its the combination of stigma and the boss wanting you to be actually desperate. If your rent is low, you're not going to appear very desperate. If you don't have a wife, kids or car to take responsibility for, you're not very desperate. If you're not desperate, you'll have the option to jobhop without fear quicker.

I've been thrown out because they didn't like me regardless whether I did a good job and I've been thrown out in a combination off 'You're not from the right school and you live with your parents'. It's disgusting and I've been jobhunting for nearly a decade now, they can do this thanks to daily contracts.