Actually, my mom discouraged my brother and I from working while we're in school. You don't make much money without a degree, and it takes time away from studying, etc. At some point after 9/11, she was laid off and ended up dipping into her retirement funds because she didn't want us to work until we were out of school. So that being said, my first "job" didn't come until senior year of college, and it was basically tutoring 2 hours a week for some extra spending cash. Of course I plan on getting a real job between graduation and med school, but that's because I no longer have heavy course loads to worry about. My brother got a summer job this year, but my mom is making him quit before he goes to school in the Fall. It's not like we can't use the extra money, but her logic is that focusing more on doing well in school now would help you get a better job in the future, and even if it didn't, you at least have a good education to fall back on. If you do work while you're in school, some people get burned out and their grades suffer, and it's not like you can make much without your degree.
I've noticed a similar trend with my friends' parents. The parents who had to hold down multiple full time jobs in college that payed minimum wage want to do everything in their power to prevent their kids from having to work, because they know it's tough to balance it. However, some of my wealthier friends were pushed to get jobs by their parents, because the parents felt it was important to learn the value of money since their kids had it whenever they wanted. So I think a large part of working while you're in undergrad depends on your circumstances, how you were raised, etc. I should also mentioned that I chose a relatively inexpensive state school (less than $10k a year including most expenses) so that I could focus on studying instead of working.