Early on in school I viewed "grades" as the goal and made decisions based on expected return on effort. If I could get a B by doing 2 hours of work a week and an A would require 10 hours then I would happily accept the B so I could screw off.

It wasn't until the end of my undergrad junior year that the goal became "education". I then focused my efforts on as learning as much as possible and gaining the best understanding.

For your specific example: If I expect a 90, but realize I could do as bad as an 80 I may still leave. Is it worth 30min of my time to get 5 points, which ends up being .1% of my final grade? I say neah.

I'm not sure I would have made it through undergrad without the co-op job breaking up the year (middle 3 years is 6months school, 6 months work).