Yeah, like DrYoda mentioned, when I suggest working, I am NOT talking about min wage campus 'work-study' programs which aren't generally worth it in terms of $ (but maybe very valuable, depending on the job, in terms of connections/experience, alternative compensation, etc such as TA, RA, LA, note-taker, peer counselor, etc.)
I worked as a CNA (certified at a summer program) in the evenings/overnights and as a welder on weekends, plus delivered newspapers and put in time as a teaching assistant, resident advisor (which also provided certificaitons and housing), note taker, lab assistant, tutor, and the OTC drug distribution at a campus infirmary. Some of those jobs 'paid' min wage, but were invaluable in terms of certifications that helped later on, or covering the cost of housing, etc.
It DID still affect my grades (I also went to a highly ranked small, private, liberal arts school....mostly because I could get more in scholarships to offset tuition enough to make it cheaper than any state school I was accepted at.) Then again, my school didn't believe in grade inflation; among science students there were zero 4.0's at graduation. We do have some of the highest acceptance rates in grad programs (including professional programs.)