How much can a senior in college with a FT offer chill in classes?

Of course grades are still important for future opportunities like PE recruiting, MBA applications, etc. but if a senior is taking some pretty challenging courses and the opportunity cost of getting all As (versus a B here or there) would be missing out on some more fun in their final year of school, what would you say is the rule of thumb for when a senior should be worried about their GPA? It may be a GPA cutoff (assume they're starting somewhere high, like 3.9-4.0 range), or it may not be.