Well, with regret, I have to say that IMO the grades do matter. Our DD is a senior this year and it has been surprising what the grade cutoffs and criteria are for scholarships and honors college admission at mid tier universities. On the high end, the GPA cut off for national merit finalist seems to be above a 3.8 (may be a 3.85). This comes from both online forums and my daughter's friends failing to progress from semi-finalist (got the good scores on PSAT and SAT) to finalist due to their academic performance. There also seems to be a hard cut off of 1-2 semester Cs and a good behavioral record is needed. For Honors Colleges the cut off seems to be a 3.5 GPA with good test scores, although there is flex here (high SAT can have slightly lower GPA and presumably the reverse). Please note that all of these are unweighted GPAs. Our experience has been that most of the universities do not look at weighting in the initial phase of admissions. As far as I can tell "holistic admissions" means meeting some threshold (3.5 GPA, 2200 SAT, athletic recruitment, etc) and then they look at the students application in depth with extracurriculars, curriculum, and trajectory. YMMV.
Since most of this is calculated in the middle of senior year, running some quick numbers on an excel spreadsheet reveals that (with a load of 6 courses per semester for 7 semesters) only one B per semester is needed to bring the GPA down to a 3.83. You can play with the numbers to see what one yearlong C will do.
Anyway. With the benefit of hindsight, I would strive for a happy medium. If Calc BC gives your DS a B how much will it impact his GPA? Do the math (sorry for the pun). Does he have all As otherwise? Is he on the borderline of an A average (3.6 GPA)? What if he gets a C for one or both semesters?
What do the schools he is looking at want? Look at the GPA of admitted freshmen. Can he take the AP calculus BC exam and do well to offset taking "just" the AP calculus AB class?
Learning to work hard to succeed is important. Learning to move on after failure is important. Sometimes I think that the most educational failures are the ones we are not anticipating.
It's kind of like having a college scholarship that requires a 3.0 GPA and progress in your declared major. If you really need the scholarship, you need to protect the GPA and take the required classes. If you don't really need it, you can take what you want and worry less about grades.
